Radar signal pre-processing to suppress surface bounce and multipath

ABSTRACT

A method and system for detecting the presence of subsurface objects within a medium is provided. In some embodiments, the imaging and detection system operates in a multistatic mode to collect radar return signals generated by an array of transceiver antenna pairs that is positioned across the surface and that travels down the surface. The imaging and detection system pre-processes the return signal to suppress certain undesirable effects. The imaging and detection system then generates synthetic aperture radar images from real aperture radar images generated from the pre-processed return signal. The imaging and detection system then post-processes the synthetic aperture radar images to improve detection of subsurface objects. The imaging and detection system identifies peaks in the energy levels of the post-processed image frame, which indicates the presence of a subsurface object.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional PatentApplication No. 61/377,319 (Attorney Docket No. 77518.8003US00) filedAug. 26, 2010, entitled “COMPUTATIONAL SYSTEM FOR DETECTING BURIEDOBJECTS IN SUBSURFACE TOMOGRAPHY IMAGES RECONSTRUCTED FROM MULTISTATICULTRA WIDEBAND GROUND PENETRATING RADAR DATA,” and U.S. ProvisionalPatent Application No. 61/377,324 (Attorney Docket No. 77518.8004US00),filed Aug. 26, 2010, entitled “DART-BASED THREAT ASSESSMENT FOR BURIEDOBJECTS DETECTED WITH A GROUND PENETRATING RADAR OVER TIME,” which areincorporated herein by reference in their entirety.

This application is related to U.S. patent application Ser. No. ______(Attorney Docket No. 77518.8004US01), filed Aug. 26, 2011, entitled“ATTRIBUTE AND TOPOLOGY BASED CHANGE DETECTION IN A CONSTELLATION OFPREVIOUSLY DETECTED OBJECTS,” U.S. patent application Ser. No. ______(Attorney Docket No. 77518.8005US00), filed Aug. 26, 2011, entitled“DISTRIBUTED ROAD ASSESSMENT SYSTEM,” and U.S. patent application Ser.No. ______ (Attorney Docket No. 77518.8006US00), filed Aug. 26, 2011,entitled “CLASSIFICATION OF SUBSURFACE OBJECTS USING SINGULAR VALUESDERIVED FROM SIGNAL FRAMES,” which are incorporated herein by referencein their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the U.S. Department of Energy andLawrence Livermore National Security, LLC, for the operation of LawrenceLivermore National Laboratory.

BACKGROUND

Many scientific, engineering, medical, and other technologies seek toidentify the presence of an object within a medium. For example, sometechnologies detect the presence of buried landmines in a roadway or afield for military or humanitarian purposes. Such technologies may useultra wideband ground-penetrating radar (“GPR”) antennas that aremounted on the front of a vehicle that travels on the roadway or acrossthe field. The antennas are directed into the ground with the soil beingthe medium and the top of the soil or pavement being the surface. GPRsystems can be used to detect not only metallic objects but alsonon-metallic objects whose dielectric properties are sufficientlydifferent from those of the soil. When a radar signal strikes asubsurface object, it is reflected back as a return signal to areceiver. Current GPR systems typically analyze the strength oramplitude of the return signals directly to identify the presence of theobject. Some GPR systems may, however, generate tomography images fromthe return signals. In the medical field, computer-assisted tomographyuses X-rays to generate tomography images for detecting the presence ofabnormalities (i.e., subsurface objects) within a body. In theengineering field, GPR systems have been designed to generate spatialimages of the interior of concrete structures such as bridges, dams, andcontainment vessels to assist in assessing the integrity of thestructures. In such images, the subsurface objects represented by suchimages tend to appear as distinct bright spots. In addition to referringto a foreign object that is within a medium, the term “object” alsorefers to any characteristic of the medium (e.g., crack in the mediumand change in medium density) that is to be detected.

Using current imaging techniques, computational systems attached toarrays that contain dozens of antenna are unable to produce radartomography images of the subsurface in real-time. A real-time systemneeds to process the return signals from successive sampling locationsof the vehicle as it travels so that, in the steady state, the returnsignals for one sampling location are processed within the time betweensamplings. Moreover, in the case of a vehicle that detects landmines, areal-time system may need to detect the presence of the landmine in timeto stop the vehicle from hitting the landmine.

SUMMARY

Real-Time System for Imaging and Object Detection with a Multistatic GPRArray

A method for detecting presence of a subsurface object in a medium isprovided. The method includes, for each of a plurality of down-tracklocations, acquiring return signals at receivers from signals emitted bytransmitters into the medium, the transmitters and receivers operatingin a multistatic mode; pre-processing the return signals to suppresseffects of extraneous signals; for each down-track location, generatinga real aperture radar image from the pre-processed return signals forthat down-track location; for each down-track location, generating asynthetic aperture radar image from the real aperture radar images ofthat down-track location and adjacent down-track locations;post-processing the synthetic aperture radar images to separate objectsin the foreground from clutter and medium in the background by filteringout effects of clutter followed by restoring spots within the filteredimages to their pre-filtered values; for each down-track location,generating a statistic based on energy of a restored spot within thefiltered image for that down-track location; identifying down-tracklocations with peaks in the generated statistics; upon identifying adown-track location with a peak, indicating the presence of a subsurfaceobject at that down-track location. The method may also be wherein themedium is ground and the transmitters and receivers form a linear arrayof transceivers. The method may also be wherein the return signals areacquired and the presence of a subsurface object is indicated inreal-time. The method may also be wherein the pre-processing of currentreturn signals includes calculating an estimated background signal for acurrent return signal based on a combination of an estimated backgroundsignal for a prior return signal for a prior down-track location thathas the same transmitter and receiver pair and a similar air gap to thecurrent return signal and an estimated gain and bias adjusted returnsignal for the prior return signal; calculating a gain and bias adjustedreturn signal for the current return signal; and calculating anestimated foreground signal for the current return signal based on adifference between the gain and bias adjusted return signal for thecurrent return signal and the estimated background signal for thecurrent return signal. The method may also be wherein the generating ofa real aperture radar image for a down-track location includesperforming a plane-to-plane propagation of the return signals togenerate a contribution image frame; generating a contribution from eachtransmitter to the real aperture image using an aperture weightingfunction to generate a weighted sum of a component of that contributionfrom each receiver; and generating the real aperture image for thatdown-track location by summing the contributions from each transmitter.The method may also be wherein the post-processing of a syntheticaperture radar image includes applying a filter to the image to generatea filtered image, the filtered image having an original spot of pixels;designating that the pixels of the original spot are within an enlargedspot and that pixels in a region adjacent to the original spot thatsatisfy restoration criterion are also within the enlarged spot; andsetting values of the pixels based on whether the pixels are in theenlarged spot. The method may also be wherein the generating of thestatistic for a down-track location includes calculating energy ofpixels in the synthetic aperture image frame for the down-tracklocation; calculating a spot energy of pixels within the restored spot;and setting the statistic based on a ratio of the spot energy to theenergy of pixels. The method may also be wherein the surface is groundand a subsurface object is a buried explosive.

A computer-readable storage device storing computer-executableinstructions for controlling a computing device to detect presence of asubsurface object in a medium is provide. The detecting of presence isperformed by a method. The method, for each of a plurality of down-tracklocations, acquires return signals at receivers from signals emitted bytransmitters into the medium, the transmitters and receiver forming alinear array of transceivers, pre-processes the return signals tosuppress effects of extraneous signals; for each down-track location,generates a synthetic aperture radar image representing a subsurfaceimage at that down-track location from the pre-processed return signals;post-processes the synthetic aperture radar images to separate objectsin the foreground from clutter and medium in the background by filteringout effects of clutter; for each down-track location, generates astatistic based on energy of a spot within the filtered image for thatdown-track location; identifies down-track locations with peaks in thegenerated statistics; and upon identifying a down-track location with apeak, indicates the presence of a subsurface object at that down-tracklocation. The method may also be wherein the medium is ground and themethod is for detecting presence of buried explosives. The method mayalso be wherein the return signals are acquired and the presence of asubsurface object is indicated in real-time. The method may also bewherein the pre-processing of current return signals includescalculating an estimated background signal for a current return signalbased on a combination of an estimated background signal for a priorreturn signal for a prior down-track location that has a similar timedelay between when the transmitter emits the signal and when a returnsignal that bounces off a surface is acquired by the receiver and anestimated gain and bias adjusted return signal for the prior returnsignal; calculating a gain and bias adjusted return signal for thecurrent return signal; and calculating an estimated foreground signalfor the current return signal based on a difference between the gain andbias adjusted return signal for the current return signal and theestimated background signal for the current return signal. The methodmay also be wherein the generating of a synthetic aperture radar imageincludes for each down-track location generating a real aperture radarimage, the generating including performing a plane-to-plane propagationof the return signals to generate a contribution image frame; generatinga contribution from each transmitter to the real aperture image using anaperture weighting function to generate a weighted sum of a component ofthat contribution from each receiver; and generating the real apertureimage for that down-track location by summing the contributions fromeach transmitter. The method may also be wherein the post-processing ofa synthetic aperture radar image includes applying a filter to the imageto generate a filtered image, the filtered image having an original spotof pixels; designating that the pixels of the original spot are withinan enlarged spot and that pixels in a region adjacent to the originalspot that satisfy restoration criterion are also within the enlargedspot; and setting values of the pixels based on whether the pixels arein the enlarged spot. The method may also be wherein the generating ofthe statistic for a down-track location includes calculating energy ofpixels in the synthetic aperture image frame for the down-tracklocation; calculating a spot energy of pixels within the restored spot;and setting the statistic based on a ratio of the spot energy to theenergy of pixels.

A computing device for detecting presence of a subsurface object in amedium from return signal acquired at a plurality of down-tracklocations by receivers from signals emitted by transmitters into themedium is provided. The transmitters and receivers operates in amultistatic mode The device includes a component that pre-processes thereturn signals to suppress effects of extraneous signals; a componentthat, for each down-track location, generates a real aperture radarimage from the pre-processed return signals for that down-tracklocation; a component that, for each down-track location, generates asynthetic aperture radar image from the real aperture radar images ofthat down-track location and adjacent down-track locations; a componentthat post-processes the synthetic aperture radar images to separateobjects in the foreground from clutter and medium in the background byfiltering out effects of clutter followed by restoring spots within thefiltered images to their pre-filtered values; a component that, for eachdown-track location, generates a statistic based on energy of a restoredspot within the filtered image for that down-track location; and acomponent that indicates the presence of a subsurface object at adown-track location having a statistic that represents a location peakin the statistics. The computing device may also be wherein thecomponent that pre-processes the return signals wherein thepre-processing includes calculating an estimated background signal for acurrent return signal based on a combination of an estimated backgroundsignal for a prior return signal for a prior down-track location thathas a similar air gap to the current return signal and an estimated gainand bias adjusted return signal for the prior return signal; calculatinga gain and bias adjusted return signal for the current return signal;and calculating an estimated foreground signal for the current returnsignal based on a difference between the gain and bias adjusted returnsignal for the current return signal and the estimated background signalfor the current return signal. The computing device may also be whereinthe component that generates a real aperture radar image for adown-track location performs a plane-to-plane propagation of the returnsignals to generate a contribution image frame; generates a contributionfrom each transmitter to the real aperture image using an apertureweighting function to generate a weighted sum of a component of thatcontribution from each receiver; and generates the real aperture imagefor that down-track location by summing the contributions from eachtransmitter. The computing device may also be wherein the component thatpost-processes a synthetic aperture radar image applies a filter to theimage to generate a filtered image, the filtered image having anoriginal spot of pixels; designates that the pixels of the original spotare within an enlarged spot and that pixels in a region adjacent to theoriginal spot that satisfy restoration criterion are also within theenlarged spot; and sets values of the pixels based on whether the pixelsare in the enlarged spot. The computing device may also be wherein thecomponent that generates the statistic for a down-track locationcalculates energy of pixels in the synthetic aperture image frame forthe down-track location; calculates a spot energy of pixels within therestored spot; and sets the statistic based on a ratio of the spotenergy to the energy of pixels.

Radar Signal Pre-Processing to Suppress Surface Bounce and Multipath

A method performed by a computing device for suppressing effects ofextraneous signals in a current return signal acquired by a receiverfrom a signal emitted by a transmitter is provided. The current returnsignal is collected at a current cross-track location of a currentdown-track location. The method includes calculating an estimatedbackground signal for the current return signal based on a combinationof an estimated background signal for a prior return signal for a priordown-track location that has a similar air gap to the current returnsignal and an estimated gain and bias adjusted return signal for theprior return signal; calculating a gain and bias adjusted return signalfor the current return signal; and calculating an estimated foregroundsignal for the current return signal based on a difference between thegain and bias adjusted return signal for the current return signal andthe estimated background signal for the current return signal. Themethod may also include calculating an adaptation parameter based ondisparity between the gain and bias adjusted return signal and theestimated background signal for the prior return signal and wherein thecalculating of the estimated background signal for the current returnsignal weights the estimated background signal for a prior return signaland the estimated gain and bias adjusted return signal for the priorreturn signal based on the adaptation parameter. The method may also bewherein the adaptation parameter decreases with increasing disparitybetween the estimated background signal for the prior return signal andthe estimated gain and bias adjusted return signal for the prior returnsignal. The method may also be wherein the disparity is a sum ofdifferences between the estimated background signal for the prior returnsignal and the estimated gain and bias adjusted return signal for theprior return signal. The method may also be wherein wherein theadaptation parameter is based on a mean of disparities of priordown-track locations and a standard deviation of disparities of priordown-track locations. The method may also be wherein the air gap is atime delay between when the transmitter emits the signal and a returnsignal that bounces off a surface is acquired by the receiver. Themethod may also be wherein the transmitter and receiver are within alinear array of transceivers that operates in a multistatic mode. Themethod may also be wherein emitted signals are emitted into a mediumhaving a surface and including, for each of a plurality of down-tracklocations, for each of a plurality of cross-track locations for eachdown-track location, calculating an estimated foreground signal for thereturn signal for that cross-track location; and detecting an objectbelow the surface of medium from the estimated foreground signals forthe return signals. The method may also be wherein the prior returnsignal was acquired by the same receiver from a signal emitted by thesame transmitter.

A computer-readable storage device storing computer-executableinstructions for controlling a computing device to suppress effects ofextraneous signals in return signals acquired by receivers fromground-penetrating radar signals emitted by transmitters operating in amultistatic mode is provided. The suppressing is performed by a methodcomprising receiving return signals for each transmitter and receiverpair at each of a plurality of down-track locations; and for eachcurrent return signal, calculating an estimated background signal forthe current return signal based on an estimated background signal for aprior return signal for a prior down-track location that has a similarair gap to the current return signal and an estimated gain and biasadjusted return signal for the prior return signal; calculating a gainand bias adjusted return signal for the current return signal; andcalculating an estimated foreground signal for the current return signalbased on a difference between the gain and bias adjusted return signalfor the current return signal and the estimated background signal forthe current return signal. The method may also include calculating anadaptation parameter based on disparity between the gain and biasadjusted return signal and the estimated background signal for the priorreturn signal and wherein the calculating of the estimated backgroundsignal for the current return signal weights the estimated backgroundsignal for a prior return signal and the estimated gain and biasadjusted return signal for the prior return signal based on theadaptation parameter. The method may also be wherein the adaptationparameter decreases with increasing disparity between the estimatedbackground signal for the prior return signal and the estimated gain andbias adjusted return signal for the prior return signal. The method mayalso be wherein the disparity is a sum of differences between theestimated background signal for the prior return signal and theestimated gain and bias adjusted return signal for the prior returnsignal. The method may also be wherein the adaptation parameter is basedon a mean of disparities of prior down-track locations and a standarddeviation of disparities of prior down-track locations. The method mayalso be wherein emitted signals are emitted into a medium having asurface and including detecting an object below the surface of themedium from the estimated foreground signals for the return signals.

A computing device for suppressing effects of extraneous signals in acurrent return signal acquired by a receiver from a signal emitted by atransmitter is provided. The signal is a ground-penetrating radar signalemitting towards the ground. The current return signal has a pluralityof samples corresponding to times at which the return signal was sampledby the receiver. The current return signal is collected at currentdown-track location. The computing device comprises a component thatgenerates an adaptation parameter based on disparity between anestimated background signal for a prior return signal for a priordown-track location that has a similar time delay between when thetransmitter emits the signal and a return signal that bounces off asurface is acquired by the receiver as the current return signal and anestimated gain and bias adjusted return signal for the prior returnsignal; a component that generates an estimated background signal forthe current return signal as a combination of the estimated backgroundsignal for the prior return signal and the estimated gain and biasadjusted return signal for the prior return signal, the combinationbeing weighted by the adaptation parameter; a component that generates again and bias adjusted return signal for the current return signal; anda component that generates an estimated foreground signal for thecurrent return signal based on a difference between the gain and biasadjusted return signal for the current return signal and the estimatedbackground signal for the current return signal. The computing devicemay also be wherein the adaptation parameter is based on a mean ofdisparities of prior down-track locations and a standard deviation ofdisparities of prior down-track locations. The computing device may alsobe wherein the transmitter and receiver are within a linear array oftransceivers that operates in a multistatic mode. The computing devicemay also include a component that for each of a plurality of down-tracklocations and for each of a plurality of cross-track locations for eachdown-track location, generates an estimated foreground signal for thereturn signal for that cross-track location; and a component thatdetects a buried object from the estimated foreground signals for thereturn signals. The computing device may also be wherein the buriedobject is an explosive device.

Spatially Adaptive Migration Tomography for Multistatic GPR Imaging

A method performed by a computing device for generating an image framefrom multistatic ground-penetrating radar (“GPR”) return signals ofmultiple transceivers is provided. The return signals are acquired bymultiple receivers from each signal emitted by each of multipletransmitters. The method comprises for each column of the image frame,identifying contributing transmitter and receiver pairs that are in aneighborhood of a transceiver that is closest to the column, theneighborhood being defined by a multistatic degree, being less than allof the transmitter and receiver pairs, and including the transmitter andreceiver pair of the transceiver that is closest to the column; andgenerating values for pixels of that column of the image frame usingcontributing samples of return signals of the contributing transmitterand receiver pairs. The method may also be wherein the multistaticdegree defines a number of adjacent transmitters and receivers that, oneither side of the transceiver that is closest to a column, that arecontributing transmitter and receiver pairs for the pixels in thecolumn. The method may also be wherein the multistatic degree isspecified by a user. The method may also include dynamically identifyingweights of the contributing transmitter and receiver pairs to a pixelbased on statistical significance of the contributing sample of thereturn signal of each contributing transmitter and receiver pair to thevalue of a pixel. The method may also be wherein the return signals forthe image frame are acquired for a down-track location and thestatistical significance of a contributing transmitter and receiver pairto the value of a pixel is based on comparison of contributing samplesof the return signal of that contributing transmitter and receiver pairto pixels of a row of the image frame that contains the pixel based on amean and standard deviation of the contributing samples of thecontributing transmitter and receiver pairs from prior down-tracklocations that contribute to pixels in the row. The method may also bewherein the statistical significance is based on an unsupervised binaryclassifier. The method may also be wherein the unsupervised binaryclassifier determines statistical significance for a pixel bycalculating a running mean and standard deviation in a left directionand a right direction of weighted contributing samples of the returnsignal of each transmitter and receiver pair, the contributing samplesbeing sorted based on magnitude of the contributing samples, selecting athreshold value from the running means and standard deviations, andindicating that transmitter and receiver pairs with magnitudes ofsamples that is greater than the threshold value are contributingtransmitter and receiver pairs. The method may also be wherein whereinwhen the identified weight for a contributing transmitter and receiverpairs to a pixel is less than a contribution threshold, discarding thecontributing sample of the return signal of that contributingtransmitter and receiver pair so that that contributing sample does notcontribute to the value of the pixel.

A computer-readable storage device storing computer-executableinstructions for controlling a computing device to generate an imageframe from multistatic ground-penetrating radar (“GPR”) return signalsof multiple transceivers is provided. The return signals are acquired bymultiple receivers from each signal emitted by each of multipletransmitters. The generating is by a method comprising, for each of aplurality of pixels in the image frame, dynamically identifyingcontributing transmitter and receiver pairs whose return signals are tocontribute to the pixel; and generating a value for the pixel from acontributing sample of the return signal of each contributingtransmitter and receiver pair. The method may also include, for each ofa plurality of pixels in the image frame, applying an image segmentationalgorithm to a matrix of weighted return signals, the matrix having arow and a column for each transmitter and receiver pair with an elementthat is a weighted contributing sample based on statistical significanceof the contributing sample of the transmitter and receiver pair to thepixel, the image segmentation algorithm identifying a region oftransmitter and receiver pairs whose weighted samples are thecontributing transmitter and receiver pairs. The method may also bewherein the contributing transmitter and receiver pairs are the same foreach pixel in a column of the image frame. The method may also bewherein the dynamically identifying of the contributing transmitter andreceiver pairs applies an unsupervised binary classifier to thecontributing sample of each transmitter and receiver pair. The methodmay also be wherein the unsupervised binary classifier identifies thecontributing transmitter and receiver pairs based on statisticalsignificance that is determined by calculating a running mean andstandard deviation in a left direction and a right direction on thecontributing sample of each transceiver and receiver pair, thecontributing samples being sorted based on magnitude of the contributingsamples, selecting a threshold value from the running means and standarddeviations, and indicating that transmitter and receiver pairs withmagnitudes of contributing samples that are greater than the thresholdvalue are contributing transmitter and receiver pairs. The method mayalso be wherein the contributing samples used by the unsupervised binaryclassifier are weighted contributing samples based on statisticalsignificance of the contributing samples to the pixel.

A computing device for generating an image frame from multistaticground-penetrating radar (“GPR”) return signals of multiple transceiversis provided. The return signals are acquired by multiple receivers fromeach signal emitted by each of multiple transmitters. The computingdevice comprising a data store that specifies a subset of transmitterand receiver pairs as contributing transmitter and receiver pairs whosereturn signals contribute to a pixel of the image frame; and a componentthat generates values for the pixel of the image frame usingcontributing samples of the return signals of the contributingtransmitter and receiver pairs. The computing device may also be whereinthe contributing transmitter and receiver pairs are in a neighborhood ofa transceiver that is closest to the pixel of the image frame, theneighborhood being defined by a multistatic degree and including thetransmitter and receiver pair of the transceiver that is closest to thecolumn. The computing device may also be wherein the multistatic degreedefines a number of adjacent transmitters and receivers, on either sideof the transceiver that is closest to a pixel, that are contributingtransmitter and receiver pairs for the pixel. The computing device mayalso include a component that dynamically identifies weights of thecontributing transmitter and receiver pairs to the pixel based onstatistical significance of the contributing sample of the return signalof each contributing transmitter and receiver pair to the value of thepixel. The computing device may also be wherein the return signals forthe image frame are acquired for a down-track location and thestatistical significance of a contributing transmitter and receiver pairto the value of the pixel is based on comparison of contributing samplesof the return signal of that contributing transmitter and receiver pairto pixels of a row of the image frame that contains the pixel based on amean and standard deviation of the contributing samples of thecontributing transmitter and receiver pairs from prior down-tracklocations that contribute to pixels in the row. The computing device mayalso be wherein the component that dynamically identifies weightincludes an unsupervised binary classifier that determines statisticalsignificance for the pixel by calculating a running mean and standarddeviation in a left direction and a right direction of weightedcontributing samples of the return signal of each transmitter andreceiver pair to the pixel, the contributing samples being sorted basedon magnitude of the contributing samples, selecting a threshold valuefrom the running means and standard deviations, and indicating thattransmitter and receiver pairs with magnitudes of samples that isgreater than the threshold value are contributing transmitter andreceiver pairs.

Zero Source Insertion Technique to Account for Undersampling in GPRImaging

A method of generating a contribution image frame for a transmitterbased on return signals acquired by multiple receivers from a signalemitted by the transmitter at a down-track location is provided. Thereceivers are at cross-track locations. The method comprises providing amatrix of the return signals with each column representing a receiverand each row representing a fast time delay; applying a fast Fouriertransform separately to each column of the matrix to generate a columntransformed matrix; applying a fast Fourier transform separately to eachrow of the column transformed matrix to generate a row transformedmatrix; inserting columns of zeros into the row transformed matrix toaccount for under-sampling in a cross-track direction to generate a zeroinserted matrix; applying an inverse fast Fourier transform separatelyto each row of the zero inserted matrix to generate an inverse rowtransformed matrix; applying an inverse fast Fourier transformseparately to each column of the inverse row transformed matrix togenerate an inverse column transformed matrix; and creating thecontribution image frame from elements of the inverse column transformedmatrix. The method may also be wherein the number of columns of zerosinserted between each column of the row transformed matrix is sufficientto ensure that the total number of columns in the cross-track directionis greater than the width of the contribution image frame divided by apixel width in an image frame. The method may also be wherein the numberof columns of zeros is based on the sampling theorem. The method mayalso be wherein the emitted signal is a ground-penetrating radar signalthat is emitted into the ground for detecting a buried explosive device.The method may also include creating a contribution image frame for eachof a plurality of transmitters at the down-track location and generatinga real aperture image for the down-track location from the createdcontribution image frames. The method may also be wherein thetransmitters and receivers operate in a multistatic mode. The method mayalso include generating real aperture images for a plurality ofdown-track locations and generating a synthetic aperture image for atarget down-track location from the real aperture images of adjacentdown-track locations.

A computer-readable storage device storing computer-executableinstructions for controlling a computing device to generate acontribution image frame for a transmitter based on return signalsacquired by multiple receivers from a signal emitted by the transmitteris provided. The receivers are at locations. The return signal for eachreceiver includes a number of samples indicating amplitude of the returnsignal. Each sample corresponds to a fast time delay at which the signalemitted by the transmitter was acquired by that receiver. The generatingis performed by a method comprising transforming the return signal ofeach receiver from a time domain into a frequency domain to generatefirst transformed return signals; transforming the first transformedreturn signals across the receivers for each fast time delay in afrequency domain to generate second transformed return signals;augmenting the second transformed return signals with zero returnsignals to represent virtual receivers to account for under-sampling bythe receivers to generate zero inserted return signals; inverselytransforming the zero inserted return signals across the receivers foreach fast time delay to generate first inverse transformed returnsignals; inversely transforming the first inverse transformed returnsignals for each receiver from the frequency domain to a time domain togenerate second inverse transformed return signals; and creating thecontribution image frame from elements of the inverse transformed returnsignals. The method may also be wherein the transforming is performedusing a fast Fourier transform and the inverse transforming is performedusing an inverse fast Fourier transform. The method may also be whereinthe number of virtual receivers is sufficient to ensure that the totalnumber of receivers is greater than a width of the contribution imageframe divided by a pixel width of an image frame. The method may also bewherein the number of virtual receivers is based on the samplingtheorem. The method may also be wherein the emitted signal is aground-penetrating radar signal that is emitted into the ground fordetecting a buried explosive device. The method may also includecreating a contribution image frame for each of a plurality oftransmitters and generating a real aperture image from the createdcontribution image frames. The method may also be wherein thetransmitters and receivers operate in a multistatic mode. The method mayalso include generating real aperture images from the contribution imageframes transmitters at cross-track locations for a plurality ofdown-track locations and generating a synthetic aperture image for atarget down-track location from the real aperture images of adjacentdown-track locations.

A computing device for generating a contribution image frame for atransmitter based on return signals acquired by multiple receivers froma signal emitted by the transmitter at a down-track location isprovided. The receivers are at cross-track locations. The computingdevice comprising a data store that stores a matrix of the returnsignals with each column representing a receiver and each rowrepresenting a fast time delay; a component that applies a fast Fouriertransform first separately to each column of the matrix and thenseparately to each row of the transformed matrix to generate a rowtransformed matrix; a component that inserts columns of zeros into therow transformed matrix to account for under-sampling in a cross-trackdirection to generate a zero inserted matrix; a component that appliesinverse fast Fourier transform first separately to each row of the zeroinserted matrix and then separately to each column of the inversetransformed zero inserted matrix to generate an inverse columntransformed zero inserted matrix; and a component that creates thecontribution image frame from elements of the inverse column transformedzero inserted matrix. The computing device may also be wherein thenumber of columns of zeros inserted between each column of the rowtransformed matrix is sufficient to ensure that the total number ofcolumns in the cross-track direction is greater than the width of thecontribution image frame divided by a pixel width in an image frame. Thecomputing device may also be wherein the emitted signal is aground-penetrating radar signal that is emitted into the ground fordetecting a buried explosive device. The computing device may alsoinclude a component that creates a contribution image frame for each ofa plurality of transmitters at the down-track location and generating areal aperture image for the down-track location from the createdcontribution image frames. The computing device may also be wherein thetransmitters and receivers operate in a multistatic mode and including acomponent that generates real aperture images for a plurality ofdown-track locations and a component that generates a synthetic apertureimage for a target down-track location from the real aperture images ofadjacent down-track locations.

Synthetic Aperture Integration Algorithm (SAI) for SAR Imaging

A method for generating a synthetic aperture radar image frame for atarget down-track location from return signals acquired by receiversfrom signals emitted by transmitters at multiple down-track locations isprovided. The return signal includes, for transmitter and receiverpairs, a number of samples indicating amplitude of the return signal.Each sample corresponds to a time at which the signal emitted by thattransmitter was acquired by that receiver. The method comprises for eachdown-track location, for each transmitter, performing a plane-to-planepropagation of the return signals acquired by receivers from the signalemitted by that transmitter at that down-track location to generate acontribution image frame; and generating a contribution from thattransmitter to the real aperture image for that down-track locationusing an aperture weighting function to generate a weighted sum of acomponent of that contribution from each receiver at that down-tracklocation; and generating a real aperture image for that down-tracklocation by summing the contributions from each transmitter; and summingthe real aperture images for successive down-track locations into asynthetic aperture image for the target down-track location. The methodmay also be wherein the aperture weighting function is based on adistance from a receiver to a transmitter. The method may also bewherein the number of down-track locations that contribute to thesynthetic aperture radar image frame is based on a height of atransmitter above a surface, a tilt angle of the transmitter relative tovertical, and a beam width of the transmitted signal. The method mayalso be wherein the emitted signals are emitted into a medium having asurface and the plane-to-plane propagation incorporates the refractiveindex of that medium. The method may also be wherein the transmittersand receivers operate in a multistatic mode. The method may also bewherein the emitted signals are emitted into a medium having a surfaceand including detecting an object below the surface of the medium fromthe synthetic aperture images generated for down-track locations. Themethod may also be wherein the object is an explosive device. The methodmay also be wherein the plane-to-plane propagation applies a fastFourier transform to a time-reversed version of a return signal.

A computer-readable storage device storing computer-executableinstructions for controlling a computing device to generate a syntheticaperture radar image frame for a target down-track location from returnsignals acquired by receivers from signals emitted by transmitters atmultiple down-track locations is provided, the transmitters andreceivers operating in a multistatic mode. The generating performed by amethod comprising for each down-track location, for each transmitter,performing a plane-to-plane propagation of the return signals acquiredby receivers from the signal emitted by that transmitter at thatdown-track location to generate a contribution image frame, wherein theplane-to-plane propagation applies a fast Fourier transform to atime-reversed version of a return signal; and generating a contributionfrom that transmitter to the real aperture image for that down-tracklocation using an aperture weighting function to generate a weighted sumof a component of that contribution from each receiver at thatdown-track location; and generating a real aperture image for thatdown-track location by summing the contributions from each transmitter;and summing the real aperture images for successive down-track locationsinto a synthetic aperture image for the target down-track location. Themethod may also be wherein the aperture weighting function is based on adistance from a receiver to a transmitter. The method may also bewherein the number of down-track locations that contribute to thesynthetic aperture radar image frame is based on a height of atransmitter above a surface, a tilt angle of the transmitter relative tovertical, and a beam width of the transmitted signal. The method mayalso be wherein the emitted signals are emitted into a medium having asurface and the plane-to-plane propagation incorporates the refractiveindex of that medium. The method may also be wherein the emitted signalsare emitted into a medium having a surface and including detecting anobject below the surface of the medium from the synthetic apertureimages generated for down-track locations. The method may also be theobject is an explosive device.

A computing device for generating a real aperture radar image for adown-track location from return signals acquired by receivers fromsignals emitted by transmitters at the down-track location is provided.The return signals include, for transmitter and receiver pairs, a numberof samples indicating amplitude of the return signal. Each samplecorresponds to a time at which the signal emitted by that transmitterwas acquired by that receiver. The transmitters and receivers operate inmultistatic mode. The computing device comprises a component thatperforms a plane-to-plane propagation of the return signals acquired byreceivers from the signal emitted by a transmitter to generate acontribution image frame; a component that generates a contribution fromeach transmitter to the real aperture image using an aperture weightingfunction to generate a weighted sum of a component of that contributionfrom each receiver; and a component that generates a real aperture imagefor that down-track location by summing the contributions from eachtransmitter. The computing device may also include a component that sumsthe real aperture image for successive down-track locations into asynthetic aperture image for a down-track location. The computing devicemay also be wherein the aperture weighting function is based on adistance from a receiver to a transmitter. The computing device may alsobe wherein the number of down-track locations that contribute to thesynthetic aperture radar image frame is based on a height of atransmitter above a surface, a tilt angle of the transmitter relative tovertical, and a beam width of the transmitted signal. The computingdevice may also be wherein the emitted signals are emitted into a mediumhaving a surface and the plane-to-plane propagation incorporates therefractive index of that medium. The computing device may also bewherein the emitted signals are emitted into ground and includingdetecting a buried explosive from the synthetic aperture imagesgenerated for down-track locations.

Spatially Assisted Down-Track Median Filter for GPR ImagePost-Processing

A method performed by a computing device for filtering a target imageframe in an associated sequence of image frames along a down-trackdirection is provided. The target image frame has pixels with values.The method comprising for target pixels of the target image frame,calculating a median filtered value for the target pixel based on thevalue of corresponding pixels in each of the image frames in theassociated sequence; and determining whether pixels in a neighborhood ofthe target pixel satisfy a filtering criterion based on values of thepixels in the neighborhood and their associated median filtered values;and after determining that the filtering criterion is satisfied,replacing the value of the target pixel with the calculated medianfiltered value. The method may also be wherein the determining includescalculating a difference between values of pixels of the target imageframe in the neighborhood of the target pixel and their respectivemedian-filtered values; and computing the minimum of the differencevalues within the neighborhood. The method may also be wherein thefiltering criterion satisfied when a minimum of difference values withinthe neighborhood is greater than zero. The method may also be whereinthe neighborhood is a rectangular area of pixels. The method may also bewherein the associated sequence of image frames is generated from returnsignals collected on a sequence of down-track locations in the vicinityof the target down-track location. The method may also be wherein thereturn signals are acquired by receivers from ground-penetrating radarsignals emitted by transmitters, the transmitters and receiversoperating in multistatic mode. The method may also be wherein theground-penetrating radar signals are emitted into the ground andincluding determining whether an explosive device is buried based onanalysis of a filtered target image frame.

A computer-readable storage device storing computer-executableinstructions for controlling a computing device to filter a target imageframe in an associated sequence of image frames along a down-trackdirection is provided. The target image frame has pixels with values.The target image frame is generated from the return signals acquired byreceivers from ground-penetrating radar signals emitted by transmitters.The filtering is performed by a method comprising: calculating a medianfiltered value for a target pixel of the target image based on the valueof corresponding pixels in the image frames in the associated sequence;determining whether pixels in a neighborhood of the target pixel satisfya filtering criterion based on values of the pixels in the neighborhoodand the median filtered values of the pixels in the neighborhood; andwhen the filtering criterion is determined to be satisfied, replacingthe value of the target pixel with the calculated median filtered value.The method may also be wherein the determining includes calculating adifference between values of pixels of the target image frame in theneighborhood and median-filtered values of pixels of the target imageframe in the neighborhood; and computing a minimum of the differencevalues within the neighborhood. The method may also be wherein thefiltering criterion satisfied when a minimum of difference values withinthe neighborhood is greater than zero. The method may also be whereinthe target image frame is represents a synthetic aperture imagegenerated from real aperture images. The method may also be wherein thetransmitters and receivers operate in multistatic mode. The method mayalso be wherein the ground-penetrating radar signals are emitted intothe ground and including determining whether an explosive device isburied based on analysis of a filtered target image frame.

A computing device for filtering a target image frame in an associatedsequence of image frames along a down-track direction is provided. Thetarget image frame has pixels with values. The target image frame isgenerated from the return signals acquired by receivers fromground-penetrating radar signals emitted by transmitters. Thetransmitters and receivers operating in a multistatic mode. Thecomputing device comprises a component that calculates a median filteredvalue for a pixel of the target image based on the value ofcorresponding pixels in the image frames in the associated sequence; acomponent that determines whether pixels in a neighborhood of a targetpixel satisfy a filtering criterion based on values of the pixels in theneighborhood and the median filtered values of the pixels in theneighborhood; and a component that when replacing the value of thetarget pixel with a value derived from values of the pixels in theneighborhood when the criterion is satisfied. The computing device mayalso be wherein the component that calculates a difference betweenvalues of pixels of the target image frame in the neighborhood andmedian-filtered values of pixels of the target image frame in theneighborhood and computes a minimum of the difference values within theneighborhood. The computing device may also be wherein the filteringcriterion satisfied when a minimum of difference values within theneighborhood is greater than zero. The computing device may also bewherein the replaced value for the target pixel in the target imageframe is the calculated median filtered value for the target pixel. Thecomputing device may also be wherein the transmitters and receiversoperate in multistatic mode. The computing device may also be whereinthe ground-penetrating radar signals are emitted into the ground andincluding determining whether an explosive device is buried based onanalysis of a filtered target image frame. The computing device may alsobe wherein the target image frame represents a synthetic aperture imagegenerated from real aperture images.

Spot Restoration for GPR Image Post-Processing

A method for restoring an original spot within a filtered image framehaving pixels with values is provided. The filtered image frame has anoriginal un-restored spot of pixels. The filtered image frame isgenerated by applying a filter to an image frame. The method comprises:identifying a restoration criterion based on the values of the pixels ofthe original spot; designating that the pixels of the original spot arewithin an enlarged spot and that pixels in a region adjacent to theoriginal spot that satisfy the restoration criterion are also within theenlarged spot; and setting values of the pixels based on whether thepixels are in the restored spot. The method may also be wherein therestoration criterion is satisfied when the value of that pixel in theimage frame prior to applying the filter is greater than a specifiedpercentile of original spot pixel values in the image frame prior tofiltering. The method may also be wherein the values of the pixels inthe restored spot are restored to their un-attenuated values from theimage frame prior to filtering. The method may also be wherein the valueof the pixels not in the restored spot are set to zero. The method mayalso be wherein the image frame is generated from return signalsacquired by receivers from signals emitted by transmitters. The methodmay also be wherein the signals are ground-penetrating radar signalsemitted into the ground and including determining whether an explosivedevice is buried based on analysis of filtered image frame with therestored original spot. The method may also be wherein the transmittersand receivers operate in multistatic mode. The method may also bewherein the filtering includes noise/clutter filtering.

A computer-readable storage device storing computer-executableinstructions for controlling a computing device to restore an originalspot within a filtered image frame having pixels with values isprovided. The restoring is performed by a method comprising: applying afilter to an image frame to generate the filtered image frame, thefiltered image frame having an original un-restored spot of pixels;designating that the pixels of the original spot are within an enlargedspot and that pixels in a region adjacent to the original spot thatsatisfy restoration criterion are also within the enlarged spot; andsetting values of the pixels based on whether the pixels are in therestored spot. The method may also be wherein the restoration criterionis satisfied when the value of a pixel in the image frame prior toapplying the filter is greater than a specified percentile of originalspot pixel values in the image frame prior to filtering. The method mayalso be wherein the values of the pixels in the restored spot arerestored to their un-attenuated values from the image frame prior tofiltering. The method may also be wherein the value of the pixels not inthe restored spot is set to zero. The method may also be wherein theimage frame is generated from return signals acquired by receivers fromsignals emitted by transmitters. The method may also be wherein thesignals are ground-penetrating radar signals emitted into the ground andincluding determining whether an explosive device is buried based onanalysis of filtered image frame with the restored original spot.

A computing device for restoring an original spot within a filteredimage frame having pixels with values is provided. The image frame isgenerated from return signals acquired by receivers from signals emittedby transmitters. The computing device comprising a component thatapplies a filter to an image frame to generate the filtered image frame,the filtered image frame having an original un-restored spot of pixels;a component that designates that the pixels of the original spot arewithin an enlarged spot and that pixels in a region adjacent to theoriginal spot that satisfy restoration criterion are also within theenlarged spot; and a component that sets values of the pixels in theenlarged spot to their values of the image frame prior to filtering andsets value of other pixels to zero. The computing device may also bewherein the restoration criterion is satisfied when the value of a pixelin the image frame prior to applying the filter is greater than aspecified percentile of original spot pixel values in the image frameprior to filtering. The computing device may also be wherein the imageframe is generated from return signals acquired by receivers fromsignals emitted by transmitters. The computing device may also bewherein the transmitters and receivers operate in a multistatic mode.The computing device may also be wherein the signals areground-penetrating radar signals emitted into the ground and includingdetermining whether an explosive device is buried based on analysis offiltered image frame with the restored original spot. The computingdevice may also be wherein the filter is a noise/clutter filter.

Buried Object Detection in GPR Images

A method for generating a statistic for a down-track location from anoriginal reconstructed image frame and a post-processed image frame isprovided. The image frames having pixels with values. The methodcomprises calculating energy of the pixels in the reconstructed imageframe; calculating a spot energy from the values of post-processed imageframe pixels within a spot; and setting the statistic based on a ratioof the spot energy to the pixel energy. The method may also be whereinthe calculated energy of the pixels is the median of the values of thepixels in the original reconstructed image frame. The method may also bewherein the statistic is generated for each image frame in a sequence ofimage frames, peaks within the sequence are detected based on thegenerated statistic for each image frame, and subsurface objectdetection is based on the heights of those peaks. The method may also bewherein a spot is greater than a minimum area and less than a maximumarea. The method may also be wherein the statistic is generated fromeach image frame in a sequence of image frames and the resultingstatistics represents a times series of the statistic. The method mayalso be wherein each peak of sufficient height in the time series ofstatistics that is separated by a minimum number of statisticsrepresents a subsurface object. The method may also be wherein thestatistic is generated for each image frame in a sequence of imageframes and the statistic is instantaneously self-detrending. The methodmay also be wherein a sequence of image frames is generated from returnsignals collected at a sequence of down-track locations from returnsignals acquired by receivers from ground-penetrating radar signalsemitted by transmitters, the transmitters and receivers operating in amultistatic mode. The method may also be wherein the ground-penetratingradar signals are emitted into the ground and including determiningwhether an explosive device is buried based on analysis of statisticsfor the sequence of image frames.

A computer-readable storage device storing computer-executableinstructions for controlling a computing device to detect an objectwithin a sequence of image frames generated for down-track locations isprovided. Each down-track location has a reconstructed image frame andpost-processed image frame. The detecting is performed by a methodcomprising: for each down-track location, calculating energy of pixelsin the reconstructed image frame for the down-track location;calculating a spot energy from pixels of the post-processed image frameof the down-track location that are within a spot on the post-processedimage frame; and setting a statistic for the down-track location basedon a ratio of the spot energy to the pixel energy; and detectingpresence of an object within the sequence of image frames based on apeak in the statistic for the down-track locations. The method may alsobe wherein the calculated energy of the pixels is the median of thevalues of the pixels in the original reconstructed image frame. Themethod may also be wherein a spot is greater than a minimum area andless than a maximum area. The method may also be wherein an object isdetected as present for each peak of sufficient height that is separatedby a minimum number of down-track locations. The method may also bewherein the statistic is instantaneously self-detrending. The method mayalso be wherein a sequence of image frames is generated from returnsignals collected at the down-track locations by receivers fromground-penetrating radar signals emitted by transmitters.

A computing device for generating a statistic for a down-track locationfrom a reconstructed image frame and a post-processed image frame isprovided. The image frames is generated from return signals acquired atthe down-track location by receivers from ground-penetrating radarsignals emitted by transmitters. The transmitters and receivers operatein a multistatic mode. The computing device comprising a component thatcalculates energy of pixels in the reconstructed image frame; acomponent that calculates a spot energy from the values ofpost-processed image frame pixels within a spot; and a component thatsets the statistic for the down-track location based on a ratio of thespot energy to the pixel energy, the statistic for determining whetheran object is buried in the ground. The computing device may also bewherein the calculated energy of the pixels is the median of the valuesof the pixels in the original reconstructed image frame. The computingdevice may also be wherein the statistic is generated for each imageframe in a sequence of image frames, peaks within the sequence aredetected based on the generated statistic for each image frame, andburied object detection is based on the heights of those peaks. Thecomputing device may also be wherein a peak of sufficient height in thestatistics that is separated by a minimum number of statisticsrepresents a buried object. The computing device may also be wherein theburied object is an explosive device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram that illustrates an arrangement of a linear array ofantennas.

FIG. 2 is a diagram that illustrates data structures storing the returnsignals collected by the imaging and detection system when inmultistatic mode in some embodiments.

FIG. 3 is a diagram that illustrates a data structure storing the imageframes generated from the return signals by the imaging and detectionsystem in some embodiments.

FIG. 4 is a diagram illustrating the overall processing of the imagingand detection system in some embodiments.

FIG. 5 is a block diagram that illustrates the generation of a syntheticaperture image.

FIG. 6 is a block diagram that illustrates the generation of a realaperture radar image using dynamic weights.

FIG. 7 is a block diagram that illustrates components of the imaging anddetection system in some embodiments.

FIG. 8 is a block diagram that illustrates each of components of theimaging and detection system in some embodiments.

FIG. 9 is a flow diagram that illustrates a “pre-process signal scans”component of the imaging and detection system in some embodiments.

FIG. 10 is a flow diagram that illustrates the processing of a “suppresssurface bounce and multipath” component of the imaging and detectionsystem in some embodiments.

FIG. 11 is a flow diagram that illustrates the processing of a “generateimage frame” component of the imaging and detection system in someembodiments.

FIG. 12 is a flow diagram that illustrates the processing of a “generatereal aperture radar image sequence” component of the imaging anddetection system in some embodiments.

FIG. 13 is a flow diagram that illustrates the processing of a “generatesynthetic aperture radar image” component of the imaging and detectionsystem in some embodiments.

FIG. 14 is a flow diagram that illustrates the processing of a“post-process image frame” component of the imaging and detection systemin some embodiments.

FIG. 15 is a flow diagram that illustrates the processing of a“differencing/clipping” component of the imaging and detection system insome embodiments.

FIG. 16 is a flow diagram that illustrates the processing of a “subtractpixel mean” component of the imaging and detection system in someembodiments.

FIG. 17 is a flow diagram that illustrates the processing of a “subtractrow mean” component of the imaging and detection system in someembodiments.

FIG. 18 is a flow diagram that illustrates the processing of a “subtractframe mean” component of the imaging and detection system in someembodiments.

FIG. 19 is a flow diagram that illustrates the processing of a“noise/clutter filtering” component of the imaging and detection systemin some embodiments.

FIG. 20 is a flow diagram that illustrates the processing of a “medianfilter” component of the imaging and detection system in someembodiments.

FIG. 21 is a flow diagram that illustrates the processing of a “restorespots” component of the imaging and detection system in someembodiments.

FIG. 22 is a flow diagram that illustrates the processing of a “detectsubsurface object” component of the imaging and detection system in someembodiments.

FIG. 23 is a flow diagram that illustrates the processing of a “generatedetection statistics” component of the imaging and detection system insome embodiments.

DETAILED DESCRIPTION

A method and system for detecting the presence of subsurface objectswithin a medium is provided. In some embodiments, the imaging anddetection system operates in a multistatic mode by using radar returnsignals associated with every possible transmitter/receiver pair withinan array of transmitter and receiver antennas. For example, when theimaging and detection system is used on a roadway (or more generally atrack), the array of transceiver antenna pairs may be mounted on avehicle that travels down the roadway. Each transceiver antenna pair isassociated with a location across the roadway, and the transceiverantenna pairs transmit signals and receive return signals at the varioussampling locations down the roadway. The imaging and detection systempre-processes return signals to suppress the effects of extraneoussignals such as antenna coupling, external interference, surface bounceand multipath, surface roughness, and so on. After preprocessing thereturn signals, the imaging and detection system generates reconstructedimages for the sampling locations. The reconstructed images represent animage of the subsurface that extends across the medium for the length ofthe array of transceiver antenna pairs and to a certain depth. Togenerate the reconstructed images, the imaging and detection systemgenerates a real aperture radar tomography image for each samplinglocation using plane-to-plane backward propagation. The imaging anddetection system then generates a synthetic aperture radar imagerepresenting the reconstructed image for a sampling location based on asequence of real aperture radar images at nearby sampling locations. Theimaging and detection system then post-processes the reconstructedimages to assist in improving the detection of objects. The imaging anddetection system may subtract out various mean values from the pixelvalues of the reconstructed images, apply various filters, and enhancethe energy of a spot within the reconstructed images. The imaging anddetection system generates a detection statistic for each pre-processedimage. For example, the detection statistic may be based on the totalenergy of the dominant spot within a post-processed image. The imagingand detection system considers the detection statistic as a time-varyingsignal with an amplitude corresponding to the time of each sampling. Thepeaks within the detection statistic indicate a subsurface object. Theimaging and detection system then applies a peak filtering detectionalgorithm within the detection statistics to identify the presence of asubsurface object. Although the imaging and detection system isdescribed primarily for processing ground-penetrating radar data todetect buried objects (e.g., landmines, pipes, and rocks), the imagingand detection system has many other applications, such as in the civilengineering and medical fields, and may use signals other thanelectromagnetic signals, such as acoustic signals.

In some embodiments, the imaging and detection system employs a lineararray of transmitter and receiver antennas for transmitting andreceiving radar signals. For example, the linear array may consist of 16transmitters T_(i) and 16 receivers R_(j) with each transmitter T_(k)and receiver R_(k) organized into a transceiver pair. The transceiversare equally spaced across the linear array. FIG. 1 is a diagram thatillustrates an arrangement of a linear array of antennas. The lineararray may be moved across a stationary surface or may be stationary withthe surface moving. For example, the linear array may be mounted on avehicle that is driven on a road to detect buried objects or may be astationary part of a medical device in which a patient is moved underthe linear array. The linear array moves in a down-track (or y)direction relative to the surface, the linear array is oriented in across-track (or x) direction, and the linear array transmits signals inthe depth (or z) direction. For example, when a linear array is mountedon a vehicle traveling on a road, the down-track is the direction oftravel, the cross-track is the direction across the road, and the depthis the direction into the road. As shown in the linear array 101, thetransmitter and receiver T_(i)R_(i) associated with a given transceiverpair are located at substantially the same cross-track location. Thelinear array has two modes of operation: multi-monostatic andmultistatic. In monostatic mode, the signal transmitted by a transmitteris received only by the receiver of that same transceiver. Themulti-monostatic mode refers to the operation of multiple transceiversof a linear array that each operate in the monostatic mode in sequence.When in multi-monostatic mode, at each down-track or sampling location,the imaging and detection system activates the transmitters of eachtransceiver in sequence across the track and collects the return signalonly at the corresponding receiver of that transceiver. The imaging anddetection system thus collects one return signal for each transceiver ateach down-track location. The multistatic mode refers to the operationof multiple transceivers of a linear array in which each transmittertransmits in sequence, but the return signal is collected by multiplereceivers, generally all the receivers. When in multistatic mode, ateach down-track location, the imaging and detection system activates thetransmitter of each transceiver in sequence and collects the returnsignal from all the receivers as illustrated in diagram 102. If thelinear array has N transceivers, then the imaging and detection systemcollects N return signals in multi-monostatic mode and N² return signalsin multistatic mode. In some embodiments, the array of transceivers maynot be linear or may be organized into a grid of transceivers.

FIG. 2 is a diagram that illustrates data structures storing the returnsignals collected by the imaging and detection system when inmultistatic mode in some embodiments. The data structures 201 store thequantized return signals. In operation, each transmitter transmits bysending a transmit signal and each receiver acquires a return signal.The receivers operate at a sampling rate and collect M return samples ofthe return signal per transmit signal. Each sample has a value of theamplitude of the return signal at the associated time it took for thetransmitted signal to be reflected back to a receiver, so earliercollected samples may indicate reflection from the surface and latercollected samples may indicate reflection off some subsurface object.The time between the transmitting of the signal and the collection of areturn sample is referred to as a “fast time delay.” When in multistaticmode and the return signal is collected from each of the N receivers,the imaging and detection system collects M×N² samples for eachdown-track location as indicated by diagram 200. The return signalscollected at each down-track location are referred to as a signal frame.

FIG. 3 is a diagram that illustrates a data structure storing the imageframes generated from the return signals by the imaging and detectionsystem in some embodiments. The imaging and detection system generatesan image frame of pixels for each down-track location that represents asubsurface image at that down-track (or y) location that spans acrossthe track and down to a certain depth. Each image frame thus representsan image that is the width of the track down to a prescribed depth. Eachcolumn of an image frame represents a certain cross-track location, andeach row of the image frame represents a certain depth. The cross-trackwidth divided by the number of columns X represents the cross-trackresolution Δ_(x) of the image frame, and the prescribed depth divided bythe number of rows Z represents the depth resolution Δ_(z) of the imageframe. The imaging and detection system thus represents each image byX×Z pixels. Each image is represented as an image frame, and the valueof each pixel represents the energy associated with the subsurface pointat the cross-track location and depth of the pixel at the down-tracklocation of the image frame.

FIG. 4 is a diagram illustrating the overall processing of the imagingand detection system in some embodiments. Signal scan grid 401represents the samples collected by the imaging and detection system atthe down-track locations. The signal scan grid has a plane for eachtransmitter and receiver pair, and the planes have a column for eachdown-track location and a row for each sample (or fast time delay). Eachcolumn represents a return signal. A down-track signal scan is thus asequence of return signals (a 2D array in which each column is a returnsignal acquired in sequence down-track), and there is one down-tracksignal scan for each transmitter and receiver pair. Conceptually, asubsurface object that is a point is represented by a hyperbolic arcacross a signal scan. When the linear array approaches that subsurfaceobject, the return signal from that subsurface object initially appearsat a relatively long fast time delay, but the fast time delay for thecorresponding return signal decreases as the linear array gets closerand increases as the linear array then gets farther away, forming ahyperbolic arc across the signal scan. A reconstruct and processsubsurface images function 404 inputs the signal scans and generates theimage frames 402 at the various down-track locations. An area of highenergy (with pixels of high value) in an image frame indicates thepresence of a subsurface object. A “generate and analyze time series ofdetection statistics” function 405 inputs the image frames and generatesdetection statistics as represented by a time-series graph 403. Theimaging and detection system generates a detection statistic value foreach down-track location such that a high detection statistic valuetends to indicate that the subsurface object is present at thatdown-track location. The time-series graph plots the detection statisticvalue for each down-track location. The peaks in the time-series graphtend to indicate the presence of a subsurface object.

Signal Pre-Processing

The imaging and detection system pre-processes the return signals tosuppress various artifacts such as those resulting from antennacoupling, external interference, surface bounce and multipath, andsurface roughness. Antenna coupling is caused by transmitted signalsthat repeatedly bounce off nearby metal antennas. The antenna couplingis characterized by large amplitude return signals at certain fast timedelays that produce “streaks” in the return signals across thedown-track locations. The effects of antenna coupling can be mitigatedby packing radar absorption material around the antennas. The imagingand detection system also suppresses the effects of antenna couplingcomputationally by subtracting out a short duration median of thesamples across the down-track location for each fast time delay. Theimaging and detection system suppresses external interference (e.g.,caused by communication transmissions) by applying a very short durationmedian filter to the samples across the track locations for each fasttime delay.

The imaging and detection system employs a recursive mean update (“RMU”)algorithm to cancel the effects of extraneous signals in the returnsignals. When a GPR signal from a transmitter arrives at the surface,part of the signal refracts into the medium and part of the signalreflects off the surface. The part of the signal that reflects off thesurface is referred to as surface bounce. When a surface bounce reachesan antenna, a portion of the surface bounce returns to the antenna arrayand is reflected back to the surface creating a multipath effect. Theprocess continues until the surface bounces attenuate. These surfacebounces are referred to as the primary surface bounce, secondary surfacebounce, and so on.

For each transmitter and receiver pair, the RMU algorithm considersreturn signals in groups for which the time delay Δ_(t) from transmitterto potentially uneven ground surface to receiver (the air gap) are allvirtually equal. The RMU algorithm estimates the background for eachreturn signal as a weighted average of previously acquired gain-biasadjusted versions of return signals that belong to the same group (i.e.,that have the same transmitter and receiver pair and air gap). The RMUalgorithm updates the background estimate by adding an adaptivelyweighted version of the gain-bias adjusted version of the previousreturn signal to an adaptively weighted version of the previousbackground estimate. The adaptation parameter is based on the disparitybetween the previous background estimate and the gain-bias adjustedversion of the previous return signal.

The RMU algorithm models the k^(th) return signal transmitted by T_(i)and received by R_(j) from the designated group by the followingequation:

g(k)=s(k)+b(k)   (1)

where g(k)={g_(l)(k)}_(l−0) ^(M−1) represents a return signal with Msamples, s(k) represents the signal of interest or the foregroundcomponent, and b(k) represents the rest of the return signal orbackground component (attributed to surface bounce, clutter, noise, andso on). The RMU algorithm estimates s(k) by the following equation:

ŝ(k)=g(k)−{circumflex over (b)}(k)   (2)

where ŝ(k) and {circumflex over (b)}(k) are estimates of s(k) and b(k),respectively. The RMU algorithm estimates the background of the returnsignal by the following equation:

$\begin{matrix}{{\hat{b}(k)} = \left\{ \begin{matrix}{g(0)} & {k = 0} \\{{\left\lbrack {1 - {\alpha (k)}} \right\rbrack {\hat{b}\left( {k - 1} \right)}} + {{a(k)}{\overset{\sim}{g}\left( {k - 1} \right)}}} & {k > 0}\end{matrix} \right.} & (3)\end{matrix}$

where {circumflex over (b)}(k) represents the estimated backgroundsignal for g(k), ĝ(k) represents a gain-bias adjusted version of g(k),and α(k) represents an adaptation parameter. The adaptation parameter isa value between 0 and 1 that quantifies summation weights for theupdated background estimate. The RMU algorithm represents the gain-biasadjusted version of g(k) by the following equation:

$\begin{matrix}{{\overset{\sim}{g}(k)} = {\begin{bmatrix}{g(k)} & 1\end{bmatrix}\begin{bmatrix}{A_{0}(k)} \\{A_{1}(k)}\end{bmatrix}}} & (4)\end{matrix}$

where {tilde over (g)}(k) represents g(k) with an optimal least-squaresgain and bias and A₀ and A₁ are represented by the following equation:

$\begin{matrix}{{\begin{bmatrix}{A_{0}(k)} \\{A_{1}(k)}\end{bmatrix} = {\begin{bmatrix}1 \\0\end{bmatrix}\left( {k = 0} \right)}},{\begin{bmatrix}{\left\lbrack {{{{\hat{b}}^{T}\left( {k - 1} \right)}{g(k)}} - {M\; {\mu_{\hat{b}}\left( {k - 1} \right)}{\mu_{g}(k)}}} \right\rbrack/{{g(k)}}^{2}} \\{\left\lbrack {{{\mu_{\hat{b}}\left( {k - 1} \right)}{{g(k)}}^{2}} - {{{\hat{b}}^{T}\left( {k - 1} \right)}{g(k)}{\mu_{g}(k)}}} \right\rbrack/{{g(k)}}^{2}}\end{bmatrix}\left( {k > 0} \right)}} & (5)\end{matrix}$

where μ_(x)(k) represents the mean of signal x, ∥g(k)∥ represents theEuclidean length of the vector represented by g(k), and M is the numberof samples in g(k). Each element of {tilde over (g)}(k) is thus thecorresponding element of g(k) times A₀ plus A₁. The RMU signalrepresents the adaptation parameter by the following equation:

$\begin{matrix}{{\alpha (k)} = {{\alpha_{0}{\exp \left( {1 - \frac{\max \left\lbrack {{d(k)},{{\mu_{d}(k)} + {\sigma_{d}(k)}}} \right\rbrack}{{\mu_{d}(k)} + {\sigma_{d}(k)}}} \right)}} \in \left\lbrack {0,\alpha_{0}} \right\rbrack}} & (6)\end{matrix}$

where α₀ is between 0 and 1, d(k) represents the disparity between{tilde over (g)}(k−1) and {circumflex over (b)}(k−1), μ_(d)(k)represents the mean of the disparities through the current down-tracklocation, and σ_(d)(k) represents the standard deviation of thedisparities through the current down-track location. The RMU algorithmrepresents the disparity by the following equation:

$\begin{matrix}{{d(k)} = \left\{ {\begin{matrix}0 & {k = 0} \\{\sum\limits_{i = 0}^{M - 1}{\max \left\lbrack {{{{\overset{\sim}{g}}_{i}\left( {k - 1} \right)} - {{\hat{b}}_{i}\left( {k - 1} \right)}},0} \right\rbrack}} & {k > 0}\end{matrix}.} \right.} & (7)\end{matrix}$

The RMU algorithm has the following characteristics. At each down-tracklocation, the complexity of the RMU algorithm is O(M). The RMU algorithmis stable in that it does not use matrix inversion and is optimal in thesense that it compares background estimated signals to optimallygain-bias adjusted signals. The RMU algorithm is spatially adaptive inthat it recursively adjusts the background estimate to varying surfacebounce and multipath over time. The RMU algorithm also suppressesanomalous signals to prevent corruption of the background estimates.

The imaging and detection system then removes the effects of surfaceroughness by effectively flattening out the surface by shifting eachcolumn such that the first row of each scan occurs at the surface. Theresulting signal scan is the pre-processed signal scan.

Imaging

The imaging and detection system generates a synthetic aperture radar(“SAR”) image for each down-track location from a sequence of adjacentreal aperture radar images represented by real aperture radar (“RAR”)image frames at nearby adjacent down-track locations. The adjacentdown-track locations typically include a range of locations slightlybefore and possibly slightly after the target down-track location forwhich the synthetic aperture radar image frame is being generated. Ateach down-track location, the imaging and detection system generates aRAR image frame by summing contributions for each of N transmitters. Thecontribution from each transmitter is reconstructed by applyingplane-to-plane propagation either to all of the associated N receivedsignals or to only some subset of those received signals. The imagingand detection system then generates the synthetic aperture radar imageframe by combining associated real aperture radar image frames foradjacent down-track locations.

The imaging and detection system generates a synthetic aperture imageframe for each down-track location from a sequence of real apertureimages generated within a synthetic aperture window that contains somenumber of down-track locations before and possibly after each targetdown-track location. The imaging and detection system employs asynthetic aperture integration (“SAI”) algorithm that is computationallyefficient. FIG. 5 is a block diagram that illustrates the generation ofa synthetic aperture image. For each down-track travel time t, the SAIalgorithm initially forms a RAR image by summing image frames{u_(i)(x,z)}_(i=1) ^(N) contributed by each transmitter {T_(i)}_(i−1)^(N). Each contributed image {u_(i)(x,z)} is reconstructed by applyingplane-to-plane propagation to the return signals {s_(i,j)}_(j=1) ^(N)from transmitter T_(i) acquired by receivers R_(j). For example, theimaging and detection system may use a plane-to-plane propagationtechnique as described in Mast, J. and Johansson, E., “Three-DimensionalGround Penetrating Radar Imaging Using Multi-Frequency DiffractionTomography,” Proceedings of the International Symposium on Optics(SPIE), 2275, 196, July 1994, which is hereby incorporated by reference.When forming the RAR image frame contribution associated withtransmitter T_(i), the SAI algorithm uses an aperture weighting functionto weight contributions from each receiver based on the distance T_(i)to that receiver. The SAI algorithm then combines the real apertureimage frames for each down-track location within the synthetic aperturewindow into synthetic aperture image frame u_(S) (x,z) for thatdown-track location.

The SAI algorithm represents the synthetic aperture image frame by thefollowing equation:

$\begin{matrix}{{u_{S}\left( {x,\left. z \middle| t_{k_{0}} \right.} \right)} = {\sum\limits_{k = {k_{\min}{(k_{0})}}}^{k_{\max}{(k_{0})}}{u_{R}\left( {x,\left. {z^{\prime}\left( t_{k} \middle| z \right)} \middle| t_{k} \right.} \right)}}} & (8)\end{matrix}$

where u_(S)(x,z|t_(k) ₀ ) represents the synthetic aperture image frameat target down-track location k₀, {u_(R)(x,z|t_(k))} represents the realaperture image frame at down-track location k, z′(t_(k)|z) representsthe depth of a pixel of the real aperture image frame that contributesto the pixel of the synthetic aperture image frame, andk_(min)(k₀)≦k₀≦k_(max)(k₀) such that k_(min) is the starting down-tracklocation and k_(max) is the ending down-track location in the syntheticaperture window. The SAI algorithm may determine the number of realaperture radar image frames before and after the target down-tracklocation that are in the synthetic aperture window based on down-tracklocations whose return signals likely include a contribution of a pointat the target down-track location factoring in the height above thesurface of the antenna array, the forward tilt angle of the antenna boresight axis from the vertical, and the beam width of the transmittedsignal. The SAI algorithm uses the depth of the pixel that contributesto the synthetic aperture image frame so that the depth satisfies thefollowing equation:

delay[(y _(k) ,h) to (y _(k) ₀ +h tan φ,z)]=delay[(y _(k) ,h) to (y _(k)+h tan φ,z′(t _(k) |z))]  (9)

where h represents the height of the antenna array above the surface, φrepresents the forward tilt angle of the antenna array from vertical,and delay(A to B) represents the time delay of radar wave propagationfrom point A to point B (through the above surface medium (e.g., air)and then through the subsurface medium (e.g., soil)).

The SAI algorithm combines the contribution image frame generated by theplane-to-plane propagation for each transmitter into the real apertureimage frame for that down-track location. The SAI algorithm weights thecontribution of each transmitter to a pixel based on the differencebetween the cross-track location of the transmitter and the cross-tracklocation of the pixel as represented by the following equation:

$\begin{matrix}{{u\left( {x,z} \right)} = {\sum\limits_{i = 1}^{N}{{w\left( {{i\; \Delta_{x}} - x} \right)}{u_{i}\left( {x,z} \right)}}}} & (10)\end{matrix}$

where w represents the aperture weighting function, Δ_(x) represents thedistance between receivers, and i represents the i^(th) transmitter. Theweighting function is a non-negative function that integrates to 1 and,in some embodiments, is a symmetrical unimodal function with a peak atx=0 that decreases or remains constant (as for a rectangular pulse) as|x| increases.

In some embodiments, the SAI algorithm factors into the generation ofthe synthetic aperture image frames the speed and direction ofpropagation of the signal through the air and the medium. The imagingand detection system estimates the refractive index of the medium by thefollowing equation:

η_(soil)=η_(soil)(λ)≈√{square root over (ε_(soil)(λ))}>1   (11)

where η_(soil) represents the refractive index and ε_(soil)(λ)represents the permittivity at wavelength λ. The imaging and detectionsystem may apply a manual focusing technique to estimate the refractiveindex. The imaging and detection system collects signals for a knownobject below the surface and then varies the refractive index until aperson deems that the object is in focus.

The imaging and detection system reconstructs images at a specificcomputed pixel resolution avoid aliasing corruption in the real apertureimages. The imaging and detection system represents the width and heightof the pixels of the images by the following equations:

Δ_(x) <c/(2f _(max))   (12)

Δ_(z) =c/(η_(soil) f _(max))   (13)

where Δ_(x) represents the width of a pixel, Δ_(z) represents the heightof a pixel, and f_(max) represents the maximum frequency of theband-pass filtered signal (e.g., 4 GHz for GPR). The height of the pixelis derived from Snell's law based on the depth resolution limit, and thewidth of the pixel is derived from the sampling theorem to avoidaliasing.

The imaging and detection system efficiently performs the plane-to-planepropagation by applying the fast Fourier transform to a time-reversedversion of the return signal. The imaging and detection systemrepresents the return signals for the transmitters at each down-tracklocation as a set of two-dimensional arrays {g (t,x)} with a column foreach receiver R_(x) and a row for each fast time delay t. The imagingand detection system applies the fast Fourier transform separately toeach column of the array, resulting in {G_(t)(f_(t),x)}. The imaging anddetection system then applies a band-pass filter separately to eachcolumn of each two-dimensional array. The imaging and detection systemfinally applies a the fast Fourier transform to each row of eachtwo-dimensional array, resulting in {G_(tx)(f_(t),f_(x))}. The realaperture radar images are aliased if the receivers are under-sampled atthe highest band-pass frequency, that is, if the number of receivers isless than the track width divided by the pixel width Δ_(x) of Equation12. To prevent aliasing, the imaging and detection system uses a zerosource insertion technique in which virtual receivers that emit notime-reversed signal are inserted between the real receivers. Theimaging and detection system inserts columns of zeros into{G_(tx)(f_(t),f_(x))} (such that the number of columns is not less thanthe track width divided by the pixel width Δ_(x)) before applying aninverse fast Fourier transform separately to each row. The reconstructedRAR image is obtained as the magnitude of this inverse Fourier transform(the pixels are all non-negative).

Imaging with Spatially Adaptive Migration

The presence of a subsurface object may only be detectible in returnsignals for transmitter and receiver pairs closest to the object. Whenthe real aperture image frame is generated from all the return signals,the values of the pixels for the object may be diluted because of theinfluence of the return signals in which the object is not detectible.In some embodiments, the imaging and detection system mitigates dilutionby generating a value for each pixel of the real aperture image from asubset of the multistatic return signals. The imaging and detectionsystem may limit the contribution of return signals to each pixel totransmitter and receiver pairs that are closest to the pixel. Thedetection system represents the subset of transmitter and receiver pairsthat contribute to a pixel by the following equation:

Ω_(MDSM(x))={(i,j): max(i _(x) −n,1)≦i,j≦min(i _(x) +n,N)}  (14)

where Ω_(MDSM(x)) represents the set of transmitter and receiver pairsthat will contribute to) pixels in column x, i represents a transmitter,j represents a receiver, n represents a multistatic degree, N representsthe number of transceivers, and i_(x) represents the index of thetransceiver that is closest to column x. This index is represented bythe following equation:

i _(x)=round (xΔ _(x)/Δ_(A))   (15)

where Δ_(x) represents the pixel width and Δ_(A) represents thecross-track distance between receivers. The multistatic degree specifiesthe number of nearest adjacent transmitters and receivers on either sideof the transmitter closest to a particular column of the image thatcontribute to value of pixels of that column. For example, when themultistatic degree is 1, then only the closest transmitter and receiverto either side will contribute to the pixels in that column, resultingin a total of no more than nine pairs of contributing transmitters andreceivers. The number n(x) of transmitter and receiver pairs thatcontribute to pixels of a column is represented by the followinginequality:

n(x)≦(2n+1)²   (16)

Table 1 illustrates the transmitter and receiver pairs that contributeto the columns that are closest to each transceiver T_(i)R_(i) whenthere are 6 transceivers and the multistatic degree is 1.

TABLE 1 $\begin{matrix}T_{1} \\T_{2} \\T_{3} \\T_{4} \\T_{5} \\T_{6}\end{matrix}\overset{R_{1}}{\left\lbrack \begin{matrix}1 \\1 \\0 \\0 \\0 \\0\end{matrix} \right.}\overset{R_{2}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}1 \\1\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{3}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{4}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{5}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{6}}{\left. \begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \right\rbrack}$ (a) $\begin{matrix}T_{1} \\T_{2} \\T_{3} \\T_{4} \\T_{5} \\T_{6}\end{matrix}\overset{R_{1}}{\left\lbrack \begin{matrix}1 \\1 \\1 \\0 \\0 \\0\end{matrix} \right.}\overset{R_{2}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}1 \\1\end{matrix} \\1\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{3}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}1 \\1\end{matrix} \\1\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{4}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{5}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{6}}{\left. \begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \right\rbrack}$ (b) $\begin{matrix}T_{1} \\T_{2} \\T_{3} \\T_{4} \\T_{5} \\T_{6}\end{matrix}\overset{R_{1}}{\left\lbrack \begin{matrix}0 \\0 \\0 \\0 \\0 \\0\end{matrix} \right.}\overset{R_{2}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\1\end{matrix} \\1\end{matrix} \\1\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{3}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\1\end{matrix} \\1\end{matrix} \\1\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{4}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\1\end{matrix} \\1\end{matrix} \\1\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{5}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{6}}{\left. \begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \right\rbrack}$ (c) $\begin{matrix}T_{1} \\T_{2} \\T_{3} \\T_{4} \\T_{5} \\T_{6}\end{matrix}\overset{R_{1}}{\left\lbrack \begin{matrix}0 \\0 \\0 \\0 \\0 \\0\end{matrix} \right.}\overset{R_{2}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{3}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\1\end{matrix} \\1\end{matrix} \\1\end{matrix} \\0\end{matrix}}\overset{R_{4}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\1\end{matrix} \\1\end{matrix} \\1\end{matrix} \\0\end{matrix}}\overset{R_{5}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\1\end{matrix} \\1\end{matrix} \\1\end{matrix} \\0\end{matrix}}\overset{R_{6}}{\left. \begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \right\rbrack}$ (d) $\begin{matrix}T_{1} \\T_{2} \\T_{3} \\T_{4} \\T_{5} \\T_{6}\end{matrix}\overset{R_{1}}{\left\lbrack \begin{matrix}0 \\0 \\0 \\0 \\0 \\0\end{matrix} \right.}\overset{R_{2}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{3}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{4}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\1\end{matrix} \\1\end{matrix} \\1\end{matrix}}\overset{R_{5}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\1\end{matrix} \\1\end{matrix} \\1\end{matrix}}\overset{R_{6}}{\left. \begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\1\end{matrix} \\1\end{matrix} \\1\end{matrix} \right\rbrack}$ (e) $\begin{matrix}T_{1} \\T_{2} \\T_{3} \\T_{4} \\T_{5} \\T_{6}\end{matrix}\overset{R_{1}}{\left\lbrack \begin{matrix}0 \\0 \\0 \\0 \\0 \\0\end{matrix} \right.}\overset{R_{2}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{3}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{4}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\0\end{matrix}}\overset{R_{5}}{\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\1\end{matrix} \\1\end{matrix}}\overset{R_{6}}{\left. \begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}\begin{matrix}0 \\0\end{matrix} \\0\end{matrix} \\0\end{matrix} \\1\end{matrix} \\1\end{matrix} \right\rbrack}$ (f)As shown in Table 1(b), when transceiver T₂R₂ is closest to a column,then transmitter and receiver pairs T₁R₁, T₁R₂, T₁R₃, T₂R₁, T₂R₂, T₂R₃,T₃R₁, T₃R₂, and T₃R₃ contribute to the pixels in that column. Asillustrated by Tables 1(a) and 1(f), the columns near the ends of theimage frame have fewer than (2n+1)² transmitter and receiver pairs thatcontribute to those columns.

The imaging and detection system may also dynamically identify weightsof the transmitter and receiver pairs for their contribution of samplevalues to a pixel of a specific column of the real aperture image frame.The imaging and detection system weights signal samples based on thestatistical significance of the samples such that less statisticallysignificant samples are weighted less than more statisticallysignificant samples. The imaging and detection system determinesstatistical significance of a sample by comparing the samples thatcontribute to the pixels in a row to the mean and standard deviation ofall those contributing samples from previous down-track locations. Theimaging and detection system may dynamically identify the transmitterand receiver pairs that contribute to each column based on thedynamically identified weights. The imaging and detection system maycalculate the weights for all receiver and transmitter pairs (or pairswithin the multistatic degree) and then discard the information forthose transmitter and receiver pairs with a very low weight to effectthe dynamic identification to improve image quality.

FIG. 6 is a block diagram that illustrates the generation of a realaperture radar image using dynamic weights. The update statisticsrecursively function 601 receives a signal frame and the priorstatistics and updates the running mean and standard deviation. Theapply penalty function 602 generates the weights for each pixel. Themean function 603 sets the value of each pixel in the real apertureradar image frame to the mean of the weighted contributions of thesamples for each transmitter and receiver pair.

The imaging and detection system sets the weight for a signal sample toa value between 0 and 1 as represented by the following equation:

w _(i,j)(x,z;y _(k))=p(g _(i,j)(x,z;y _(k)); μ_(g,k)(z), σ_(g,k)(z))  (17)

where w_(i,j)(x,z;y_(k)) represents the weight for the contribution ofthe signal of transmitter and receiver pair T_(i)R_(j) to the pixel inrow z and column x of the real aperture image frame, μ_(g,k)(z)represents a running mean at down-track location k of the samples thatcontribute to row z, σ_(g,k)(z) represents a running standard deviationat down-track location k of the samples that contribute to row z,g_(i,j)(x, z;y_(k)) represents the sample at down-track location kcontributed by the transmitter and receiver pair T_(i)R_(j) to the pixelat row z and column x, and p represents a penalty function. The imagingand detection system represents the penalty function by the followingequation:

$\begin{matrix}{{p\left( {{g;\mu_{g}},\sigma_{g}} \right)} = \left\{ \begin{matrix}0 & {{{g - \mu_{g}}} \leq {n_{L}\sigma_{g}}} \\1 & {{{g - \mu_{g}}} \geq {n_{H}\sigma_{g}}} \\\frac{{g - \mu_{g} - {n_{L}\sigma_{g}}}}{\left( {n_{H} - n_{L}} \right)\sigma_{g}} & {otherwise}\end{matrix} \right.} & (18)\end{matrix}$

where g represents the signal sample and n_(L) and n_(H) represent therange of numbers of standard deviations from the mean of the samplevalues over which the weight varies linearly. The imaging and detectionsystem perform RAR imaging using the following equation:

$\begin{matrix}{{u_{R}\left( {x,{z;y_{k}}} \right)} = {\underset{{({i,j})} \in \Omega_{{MDSM}{(x)}}}{mean}\left\lbrack {{w_{i,j}\left( {x,z,y_{k}} \right)} \cdot {g_{i,j}\left( {x,{z;y_{k}}} \right)}} \right\rbrack}} & (19)\end{matrix}$

where u_(R)(x,z;y_(k)) represents the pixel value in the RAR image forcross-track location x, depth z, and down-track location y_(k). In someembodiments, the imaging and detection system may set the weights tovary in a non-linear fashion. The imaging and detection systemrepresents the running mean of the signals that contribute to the pixelsin a row by the following equation:

$\begin{matrix}{{u_{g,k}(z)}\overset{\Delta}{=}{\underset{\underset{l = {0\mspace{11mu} \ldots \mspace{11mu} k}}{{({i,j})} \in \Omega_{{MDSM}{(x)}}}}{\underset{x}{mean}}{g_{i,j}\left( {x,{z;y_{l}}} \right)}}} & (20)\end{matrix}$

The running variance (from which the running standard deviation iscalculated) of the signals that contribute to the pixels in a row isderived by the following equation:

$\begin{matrix}{{\sigma_{g,k}^{2}(z)}\overset{\Delta}{=}{\underset{\underset{l = {0\mspace{11mu} \ldots \mspace{11mu} k}}{{({i,j})} \in \Omega_{{MDSM}{(x)}}}}{\underset{x}{var}}{g_{i,j}\left( {x,{z;y_{l}}} \right)}}} & (21)\end{matrix}$

The imaging and detection system calculates the running mean of samplesin Equation 20 at current down-track location k for each row zrecursively by adding the mean computed for the current down-tracklocation to the sum of means computed for prior down-track locations andcomputing the mean of the result. The imaging and detection systemsimilarly calculates the running mean of squared samples at currentdown-track location k for each row z recursively. The imaging anddetection system then calculates the running variance of samples inEquation 21 at current down-track location k for each row z as therunning mean of squared samples minus the square of the running mean ofsamples.

In some embodiments, the imaging and detection system assesses thestatistical significance of transmitter and receiver pair contributionsto a pixel using an unsupervised binary classifier, thereby eliminatingthe need to choose a value for the multistatic degree n of Equation 14.The imaging and detection system retrieves the sample from eachtransmitter and receiver pair that contributes to a particular pixel fora total of N² samples represented by the set {s_(k)}. The imaging anddetection system then sorts the samples. The imaging and detectionsystem then generates a set of the running means and variances from boththe left and right directions from the set {s_(k)}. The set of runningmeans generated from the left direction would have an element for eachof the samples with each element being the mean of the correspondingsample and all samples to the left. The set of running means generatedfrom the right direction would have an element for each of the sampleswith each element being the mean of the corresponding sample and allsamples to the right. The imaging and detection system identifies thevalue s* representing the threshold value of a statistically significantsample as the first (or lowest element) in the set {s_(k)} thatsatisfies the condition of the following equation:

s _(k)−μ_(LK)>μ_(RK) −s _(k) and μ_(LK)−μ_(RK)>σ_(LK)+σ_(RK)   (22)

where s_(k) represents a sample, {μ_(LK)} represents the set of meansfrom the left direction, {μ_(LK)} represents the set of means from theright direction, {σ_(LK) ²} represents the set of variances from theleft direction, and {σ_(LK) ²} represents the set of variances from theright direction. If there exists an s* value that satisfies thecondition of equation 22, the imaging and detection system considers allsamples with a value greater than s* as statistically significant anduses only those samples to generate the pixel of the real aperture radarimage frame. If s* does not exist, then the imaging and detection systemgenerates the pixel from all the samples.

In some embodiments, the imaging and detection system may automaticallydetermine on a per-pixel or per-column basis which of the transmitterand receiver pairs T_(i)R_(j) to use in computing the values forindividual pixels. To identify the transmitter and receiver pairs, theimaging and detection system applies a segmentation algorithm to the N×Narrays of weighted return samples. The imaging and detection system mayweight the return samples using the weights as specified by Equation 17.The imaging and detection system maintains one N×N array for each pixelor column. The imaging and detection system may apply a standardsegmentation algorithm based, for example, on region growing or thewatershed principle, to identify portions (or segmented regions) of theN×N array, that are populated by the transmitter and receiver pairsT_(i)R_(j), that are to contribute to the pixel values of a realaperture radar image. Suitable segmentation algorithms are well-knownsuch as those described Meyer, F. and Maragos, P., “MultiscaleMorphological Segmentations based on Watershed, Flooding, and EikonalPDE,” Scale-Space ‘99, LNCS 1662, pp. 351-362, published bySpringer-Verlag Berlin Heidelberg 1999.

Image Post-Processing

The imaging and detection system processes the reconstructed imageframes (which, by construction, only contain pixels with non-negativevalues) to attenuate the effects of surface bounce and other sources ofclutter. The imaging and detection system processes a reconstructedimage frame by background subtraction, filtering of noise and clutter,and restoration of the spot. To separate subsurface objects of limiteddown-track extent in the foreground from background, the imaging anddetection system optionally subtracts background from each reconstructedimage frame u_(k) (for down-track location k) by subtracting the mean ofcorresponding pixels from previous frames from each pixel in the currentframe and setting negative differences to zero. The imaging anddetection system represents the resulting pixel-wise subtracted imageframe as {dot over (u)}_(k) for down-track location k. High residualpixel magnitudes from pixel-wise subtraction indicate the possiblepresence of a subsurface object. To separate subsurface objects oflimited cross-track extent in the foreground from background, theimaging and detection system further optionally subtracts backgroundfrom each row by subtracting the mean plus some multiple of the standarddeviation of pixels on a given row of the current image frame from allpixels on that row (i.e., at the same depth), and sets negativedifferences to zero. The imaging and detection system uses a defaultmultiple of 1 on the standard deviation, which can be overridden. Highresidual pixel magnitudes from row-wise subtraction indicate thepossible presence of a subsurface object. Row-wise subtraction alsopartially compensates for attenuation in pixel amplitudes withincreasing depth.

To further separate objects in the foreground from background, theimaging and detection system optionally subtracts the background fromeach frame by subtracting from each pixel in the current frame (afteroptional pixel-wise and row-wise subtraction), the running mean plussome multiple (a default of 1, which can be overridden) of the runningstandard deviation of pixel values in image frames (after optionalpixel-wise and row-wise subtraction). The imaging and detection systemsets negative differences to zero. In this way, the imaging anddetection system may consider a pixel to be statistically insignificantand set it to zero if its value is less than the running mean plus adesignated number of running standard deviations. The imaging anddetection system represents the resulting clipped image frame as u_(k)′for down-track location k.

The imaging and detection system employs a combination of filters tosuppress clutter and noise (e.g., speckle noise) from the clipped imageframe. The imaging and detection system applies a first segmentationfilter to the clipped image frame that sets all pixels to zero exceptthose in the region of highest total value or energy (i.e., the spot).This first segmentation filter segments the clipped image frame intoregions of non-zero pixels using a standard region growing algorithmwith a default neighborhood that can be overridden. The firstsegmentation filter then sets all pixels to zero except those in theregion with the highest energy. If a subsurface object is present, itwill typically be correspond to the region with the highest energy. Theimage frame produced by the first segmentation filter is represented asu_(k)″ for down-track location k.

The imaging and detection system then applies to each pixel of the firstfiltered image frame u_(k)″ a median filter of duration 2m+1 that isderived from frames {u_(k)″}_(j=k−m) ^(k+m), resulting in a secondfiltered image frame. In one embodiment, the imaging and detectionsystem may apply a median filter that is derived from that same pixel atprior down-track locations. Such a median filter may, however, suppressthe pixels too heavily. In some embodiments, the imaging and detectionsystem applies a spatially assisted median filter to each pixel that isderived not only from that pixel at other down-track locations but alsofrom pixels in a neighborhood (e.g., a rectangular neighborhood) of thatpixel. The spatially assisted median filter allows a pixel to be medianfiltered only when the value of every non-zero pixel in its neighborhoodis at least as large as its median. The spatially assisted median filterapplied to each pixel to generate a second filtered image is representedby the following algorithm:

Algorithm 1 1. μ_(k−m)(x, z) = median{u_(j) ^(″)(x, z)}_(j=k−2m) ^(k) 2.Δ_(k−m)(x, z) = u_(k−m) ^(″)(x, z) − μ_(k−m)(x, z) 3. Ω_(k−m)(x, z) ={(x′, z′) ε neighborhood (x, z):u_(k−m) ^(″)(x′, z′) > 0} 4.${\Delta_{k - m}^{*}\left( {x,z} \right)} = {\min\limits_{{({x^{\prime},z^{\prime}})} \in {\Omega_{k - m}{({x,z})}}}{\Delta_{k - m}\left( {x^{\prime},z^{\prime}} \right)}}$5. ${v_{k - m}^{\prime}\left( {x,z} \right)} = \left\{ \begin{matrix}{u_{k - m}^{''}\left( {x,z} \right)} & {{\Delta_{k - m}^{*}\left( {x,z} \right)} \leq 0} \\{\mu_{k - m}\left( {x,z} \right)} & {otherwise}\end{matrix} \right.$Step 1 of algorithm 1 calculates the median energy of the pixels fromthe first segmentation filtered image frames for m track locationsbefore and after the target down-track location k−m. The number of tracklocations m has a default value which may be overridden by the user.Step 2 calculates the difference between each pixel from the firstsegmentation filtered image frame at down-track location k−m and itsmedian value. Step 3 identifies a neighborhood around each pixel of thefirst filtered image frame of the target down-track location such thateach pixel in the neighborhood is greater than zero, whereneighborhood(x,z) has a default specification that can be overridden.Step 4 identifies the minimum of the differences as calculated in step 2of the pixels in the neighborhood of the pixel. Step 5 sets the pixel ofthe second filtered image frame to the value of the first filtered imageframe u_(k)″ if the minimum difference is not positive and to the medianenergy otherwise. The result of the algorithm is that a non-zero pixelof the first segmentation filtered image frame is replaced by its medianonly when all the pixels in its neighborhood are larger than theirrespective medians.

The imaging and detection system then applies a depth filter to theresulting filtered image frame to remove areas of energy with thicknessin the depth direction that are essentially too small to represent anobject. A minimal thickness of Δ_(z) pixels in the depth direction isspecified by a default value that can be overridden. The imaging anddetection system processes each column of the filtered image frameseparately. If a pixel is non-zero, then the imaging and detectionsystem sets that pixel to zero when the number of pixels of value zeroin that column within ±Δ_(z) rows of the current row exceeds Δ_(z).

The imaging and detection system then applies a second segmentationfilter (identical to the first) to the resulting filtered image frame,resulting in a spot filtered image frame represented as v_(k−m)′. Theresult is an image in which only the pixels in the spot (if any) arenonzero.

The imaging and detection system then enlarges the spot in v_(k−m)′,using Algorithm 2. The imaging and detection system then restores thevalues of the pixels in the enlarged spot to their un-attenuated valuesfrom the pixel-wise subtracted image frame {dot over (u)}_(k−m), therebyincreasing the probability of detecting a subsurface object.

Algorithm 2 1.${u_{crit}(\rho)} = {\underset{{({x,z})} \in {{spot}{(v_{k}^{\prime})}}}{{percentile}\mspace{14mu} \rho}\mspace{14mu} {{\overset{.}{u}}_{k}\left( {x,z} \right)}}$v_(k)′ = {dot over (u)}_(k) v_(k)′(x, z) = 1 + u_(crit)(ρ) ∀(x, z) ∈spot(v_(k)′) 2. (x_(seed), z_(seed)) = first element of spot (v_(k)′)spot(v_(k)) = regionGrow(v_(k)′, (x_(seed), z_(seed)) ,u_(crit)(ρ),neighborhood) 3. ${v_{k}\left( {x,z} \right)} = \left\{ \begin{matrix}{{\overset{.}{u}}_{k}\left( {x,z} \right)} & {\left( {x,z} \right) \in {{spot}\left( v_{k} \right)}} \\0 & {otherwise}\end{matrix} \right.$Step 1 of algorithm 2 calculates a default percentile ρ (which may beoverridden) of pixel values from {dot over (u)}_(k) that belong to thespot extracted from v_(k)′. Step 2 temporarily sets v_(k)′ to {dot over(u)}_(k) and then sets each spot pixel value to the value plus one. Step2 then enlarges the spot in the temporary image frame v_(k)′ toencompass neighboring pixels whose values are larger than the percentilevalue. The resulting region will include all the pixels of the originaland may be augmented with additional neighboring pixels, resulting in afully enlarged spot. Step 3 forms the post-processed image frame v_(k)by assigning un-attenuated value of pixels from {dot over (u)}_(k) tothe corresponding pixels in the fully enlarged spot.

Object Detection

The imaging and detection system extracts a detection statistic fromeach post-processed image frame v_(k), transforming the sequence ofpost-processed image frames into a time series of detection statisticvalues. An object is said to be detected in post-processed image framesfor which the detection statistic value exceeds a prescribed detectionthreshold. In practice, time series of detection statistic values canexhibit upward trends that are correlated with surface roughness, andwhich are also sensitive to surface type, soil type, soil moisturecontent, etc. These trends can dramatically reduce detection-false alarmrate performance. Time series are normally detrended by subtracting amoving average from each time series sample. However, buried objectdetection is inherently causal, that is detection decisions preferablyshould be made before vehicles drive over buried objects. In this case,the moving average window should trail the current vehicle locationdown-track, which means that the moving window applied to a specifictime series sample should trail that sample. Classical time seriesdetrending algorithms based on causal moving averages are thus typicallyunable to react quickly to sudden changes in surface or soil conditions.Worse yet, they often produce artifacts at locations of sudden change inthe surface that can be incorrectly interpreted as buried objects.

In some embodiments, the imaging and detection system addresses thisproblem by using an instantaneously self-detrending detection statistic,referred to herein as the spot ratio r_(k) defined by the followingequation:

$\begin{matrix}{r_{k}\overset{\Delta}{=}\left\{ \begin{matrix}{e_{k}/c_{k}} & {c_{k} > 0} \\0 & {c_{k} = 0}\end{matrix} \right.} & (23)\end{matrix}$

where e_(k) represents the spot energy (the sum of pixels values inpost-processed image frame v_(k) that lie within the spot) and c_(k)represents the level of frame clutter (the median over all pixel valuesin the reconstructed image frame u_(k) that v_(k) was derived from). Thee_(k) component of spot ratio is sensitive to buried objects, as buriedobjects are highly correlated with bright spots in post-processed imageframes. The e_(k) component is also sensitive to roughness in thesurface, as surface roughness can also be somewhat correlated withbright spots. Since the c_(k) component of spot ratio is highlycorrelated with surface roughness, it provides an effective detrendingmechanism for the time series {r_(k)} of spot ratio values.

The imaging and detection system may apply a size filter to the spot inthe post-processed image to remove spots that are too small or too largeto represent a subsurface object. The user may specify the minimum andmaximum admissible areas (sizes) for spots. The imaging and detectionsystem multiplies the detection statistic by 1 for spots within therange of minimum and maximum areas and by a penalty factor that maydecrease linearly for areas outside the minimum and maximum areas. Therate of decrease may be specified by a user.

The imaging and detection system applies a local peak filter to the timeseries of detection statistic values to remove multiple peaks that mayrepresent the same object and to reduce ambiguity as to the location ofa subsurface object. The imaging and detection system allows the user tooverride the default minimum number of admissible track locationsbetween peaks. The imaging and detection system then detects an objectwhen the detection statistic exceeds a peak threshold. The peakthreshold may be determined empirically based on samples collected fromknown subsurface objects.

FIG. 7 is a block diagram that illustrates components of the imaging anddetection system in some embodiments. Because some of the processing ofan image frame for one down-track location may rely on image frames oflater down-track locations, the imaging and detection system may delaysome of its processing until signal frames have been collected forseveral subsequent down-track locations. In addition, the imaging anddetection system may not perform the peak detection once for everydown-track location, but rather may generate the detection statisticsfor a small range of subsequent down-track locations and then performpeak detection once for the entire range. The imaging and detectionsystem 700 receives signal frames and stores them in a signal framestore 701. A “pre-process signal frames” component 706 inputs the signalframes, pre-processes the signal frames, and stores the pre-processedsignal frames in a pre-processed signal frame store 702. A “generateimage frames” component 707 inputs the pre-processed signal frames,generates the reconstructed image frames, and stores the reconstructedimage frames in a reconstructed image frame store 703. A “post-processimage frames” component 708 inputs the reconstructed image frames,performs the post-processing of the reconstructed image frames, andstores the post-processed image frames in a post-processed image framestore 704. A “detect objects” component 709 inputs the post-processedimage frames, generates a detection statistic for each frame, identifiespeaks with the detection statistics, and when a peak of sufficientheight is detected, outputs that an object has been detected.

FIG. 8 is a block diagram that illustrates each of the components of theimaging and detection system in some embodiments. As described above, animaging and detection system 800 includes a “pre-process signal frames”component 810, a generate image frames component 820, a “post-processimage frames” component 830, and a “detect objects” component 840. The“pre-process signal frames” component includes a “decouple antenna”component 811, a “remove interference” component 812, a “suppresssurface bounce and multipath” component 813, and a “flatten surface”component 814. The “generate image frames” component includes a“generate real aperture radar imagesequence” component 821 and a“generate synthetic aperture radar image” component 822. The “generatereal aperture radar image sequence” component includes a “calculateweights” component 823, a “calculate mean and standard deviation”component 824, an “unsupervised binary classifier” component 826, and asegmentation component 827. The “calculate weights” component calculatesthe weight for samples associated with specific transmitter and receiverpairs based on statistical significance as illustrated in FIG. 6 andEquations 17-21. The “unsupervised binary classifier” component assessesthe statistical significance of samples associated with specifictransmitter and receiver pairs by Equation 22 and accompanying text. Thesegmentation component applies a segmentation algorithm to identify thetransmitter and receiver pairs that contribute samples to each pixel orcolumn of the real aperture radar image. The “post-process image frames”component includes a “differencing and clipping” component 831, a“noise/clutter filtering” component 832, and a “restore spots” component833. The “detect objects” component includes a “generate detectionstatistics” component 841 and an “identify peaks” component 842.

The computing devices on which the imaging and detection system may beimplemented may include a central processing unit and memory and mayinclude input devices (e.g., keyboard and pointing devices), outputdevices (e.g., display devices), and storage devices (e.g., diskdrives). Computer-readable media include computer-readable storage mediaand data transmission media. The computer-readable storage media includememory and other storage devices that may have recorded upon or may beencoded with computer-executable instructions or logic that implementthe imaging and detection system. The data transmission media is mediafor transmitting data using signals or carrier waves (e.g.,electromagnetism) via a wire or wireless connection. Various functionsof the imaging and detection system may also be implemented on devicesusing discrete logic or logic embedded as an application-specificintegrated circuit. The imaging and detection system may be implementedon a computer system that is local to a vehicle to which the lineararray of antennas is mounted for processing the return signals locally.Alternatively, one or more of the components may be implemented on acomputer system that is remote from the linear array. In such analternative, the data used by the various components (e.g., returnsignals and image frames) may be transmitted between the local computingsystem and remote computer system and between remote computing systems.

The imaging and detection system may be described in the general contextof computer-executable instructions, such as program modules, executedby one or more computers or other devices. Generally, program modulesinclude routines, programs, objects, components, data structures, and soon that perform particular tasks or implement particular abstract datatypes. Typically, the functionality of the program modules may becombined or distributed as desired in various embodiments.

FIG. 9 is a flow diagram that illustrates the processing of the“down-track signal scan pre-processing” component of the imaging anddetection system in some embodiments. A down-track signal scan is asequence of return signals (a 2D array in which each column is a returnsignal acquired in sequence down-track) with one down-track signal scanfor each transmitter and receiver pair. The component applies variousprocessing techniques to the raw signal scan, resulting in thepre-processed signal scan. In block 901, the component suppresses theeffects of antenna coupling by subtracting out of the signal frame ashort duration weighted average of amplitudes of each row. In block 902,the component suppresses external interference by applying to each rowof the signal frame a very short duration median filter. In block 903,the component invokes the “suppress surface bounce and multipath”component to process the signal frame. In block 904, the componentflattens the surface by shifting each column such that the first row ofeach scan occurs at the surface. The component then returns.

FIG. 10 is a flow diagram that illustrates the processing of the“suppress surface bounce and multipath” component of the imaging anddetection system in some embodiments. The component implements the RMUalgorithm to suppress extraneous signals as described above that isapplied to the pre-processed signals from the same transmitter andreceiver pair that all share the same air gap (to within a finetolerance). The component performs the processing of blocks 1005-1011for return signals associated with each transmitter and receiver pairthat share the same air gap. In block 1001, the component selects thenext air gap. In decision block 1002, if all the air gaps have alreadybeen selected, then the component returns, else the component continuesat block 1003. In block 1003, the component selects the next transmitterand receiver pair with the selected air gap. In block 1004, if all thetransmitter and receiver pairs for the selected air gap have alreadybeen selected, then the component loops to block 1001 to select the nextreceiver, else the component continues at block 1005. In block 1005, thecomponent calculates the disparity for the selected air gap andtransmitter-receiver pair according to Equation 7. In block 1006, thecomponent calculates the mean and standard deviation of the disparitiesof past pre-processed signals and the current pre-processed signal forthe selected transmitter and receiver pair. In block 1007, the componentcalculates the adaptation parameter for the selected transmitter andreceiver pair according to Equation 6. In block 1008, the componentcalculates a gain-bias vector for the selected transmitter and receiverpair according to Equation 5. In block 1009, the component generates thegain-bias adjusted signal for the selected transmitter and receiver pairaccording to Equation 4. In block 1010, the component generates theestimated background signal within the current pre-processed signal forthe selected transmitter and receiver pair according to Equation 3. Inblock 1011, the component generates the estimated foreground signalwithin the current pre-processed signal for the selected transmitter andreceiver pair according to Equation 1. The component then loops to block1003 to select the next transmitter for the selected receiver.

FIG. 11 is a flow diagram that illustrates the processing of a “generateimage frame” component of the imaging and detection system in someembodiments. In block 1101, the component invokes the “generate RARimage sequence” component to generate the real aperture radar image forthe current down-track location based on the pre-processed signal frame.In block 1102, the component invokes the “generate synthetic apertureradar image” component to generate the synthetic aperture radar image(i.e., the reconstructed image frame) based on a set of real apertureradar images corresponding to down-track locations adjacent to thecurrent down-track location. The component then returns.

FIG. 12 is a flow diagram that illustrates the processing of the“generate real aperture radar image sequence” component of the imagingand detection system in some embodiments. In block 1201, the componentselects the next transmitter. In decision block 1202, if all thetransmitters have already been selected, then the component returns,else the component continues at block 1203. In block 1203, the componentperforms plane-to-plane propagation for the selected transmitter togenerate a contribution image frame that contains the contribution ofthe selected transmitter to the real aperture radar image for thecurrent down-track location. In block 1204, the component aggregates thecontribution image frame into the real aperture image frame and thenloops to block 1201 to select the next transmitter. The component mayaggregate the contribution image frame using a weight for each pixel inaccordance with Equation 10. In some embodiments, the component may alsouse a spatially adaptive migration technique as described above whengenerating the real aperture radar image frame. For example, thecomponent may use a fixed multistatic degree to identify the transmitterand receiver pairs that contribute to a column of the real apertureradar image frame. The component may also use a segmentation algorithmor an unsupervised binary classifier to determine the transmitter andreceiver pairs and their weights that contribute to the pixels of a realaperture radar image.

FIG. 13 is a flow diagram that illustrates the processing of the“generate synthetic aperture radar image” component of the imaging anddetection system in some embodiments. The component inputs the realaperture radar image frames and outputs the corresponding syntheticaperture radar image frame. In block 1301, the component selects thenext down-track location that is within the window for generating thesynthetic aperture radar image. In decision block 1302, if all suchdown-track locations have already been selected, then the componentreturns, else the component continues at block 1303. In block 1303, thecomponent selects the next pixel of an image frame. In decision block1304, if all the pixels have already been selected, then the componentloops to block 1301, else the component continues at block 1305. Inblock 1305, the component adds to the selected pixel of the syntheticaperture radar image frame a corresponding pixel of the selected realaperture radar image frame and then loops to block 1303 to select thenext pixel. The corresponding pixel from the real aperture radar imageis specified in accordance with Equation 9.

FIG. 14 is a flow diagram that illustrates the processing of the“post-process image frame” component of the imaging and detection systemin some embodiments. The component inputs the reconstructed image frameand outputs the post-processed image frame. In block 1401, the componentinvokes the “differencing/clipping” component to generate a clippedimage frame. In block 1402, the component invokes the “noise/clutterfiltering” component to generate the filtered image frame. In block1403, the component invokes the “restore spots” component to generatethe post-processed image frame and then returns.

FIG. 15 is a flow diagram that illustrates the processing of the“differencing/clipping” component of the imaging and detection system insome embodiments. The component inputs the reconstructed image frame andoutputs the clipped image frame. In blocks 1501 and 1502, the componentinvokes a “subtract pixel mean” component and a subtract row meancomponent to generate a differenced image frame from the reconstructedimage frame. In block 1503, the component invokes a “subtract framemean” component to generate a clipped image frame and then returns.

FIG. 16 is a flow diagram that illustrates the processing of the“subtract pixel mean” component of the imaging and detection system insome embodiments. In block 1601, the component selects the nextcross-track location. In decision block 1602, if all the cross-tracklocations have already been selected, then the component returns, elsethe component continues at block 1603. In block 1603, the componentselects the next row of the image frame. In decision block 1604, if allthe rows have been selected, then the component loops to block 1601,else the component continues at block 1605. In block 1605, the componentcalculates the mean of the pixels at the selected cross-track locationand the selected row for past reconstructed image frames. In block 1606,the component sets the pixel of the differenced image frame to thedifference between the calculated mean and the corresponding pixel ofthe current reconstructed image frame and then loops to block 1603 toselect the next row.

FIG. 17 is a flow diagram that illustrates the processing of the“subtract row mean” component of the imaging and detection system insome embodiments. In block 1701, the component selects the next row ofthe image frame. In decision block 1702, if all the rows have alreadybeen selected, then the component returns, else the component continuesat block 1703. In block 1703, the component calculates the mean andstandard deviation for the selected row of the pixel-wise differencedimage frame. In block 1704, the component selects the next column. Indecision block 1705, if all the columns have already been selected, thenthe component loops to block 1701 to select the next row, else thecomponent continues at block 1706. In block 1706, the componentsubtracts the mean plus some multiple of the standard deviation from thepixel at the selected row and column of the differenced image frame andthen loops to block 1704 to select the next column.

FIG. 18 is a flow diagram that illustrates the processing of the“subtract frame mean” component of the imaging and detection system insome embodiments. The component subtracts the frame mean from the pixelsof the pixel-wise and row-wise differenced image frame, resulting in aclipped image frame. In block 1801, the component updates the runningmean and the standard deviation for the pixel-wise and row-wisedifferenced image frame. In block 1802, the component subtracts the meanplus some multiple of the standard deviation from each pixel of thedifferenced image frame, and then sets the negative differences to zero,to generate the clipped image frame. The multiple of the standarddeviation that is used throughout the imaging and detection system maybe user-specified and may be any positive or negative number. Thecomponent then returns.

FIG. 19 is a flow diagram that illustrates the processing of the“noise/clutter filtering” component of the imaging and detection systemin some embodiments. The component inputs the clipped image frame andoutputs the post-processed image frame. In block 1901, the componentapplies a first segmentation filter to the clipped image frame,resulting in the first filtered image frame. In block 1902, thecomponent invokes the median filter component to generate the secondfiltered image frame. In block 1903, the component applies a depththickness filter to remove thin spots. In block 1904, the componentapplies a a second segmentation filter to the filtered image frame togenerate a post-processed image frame and then returns.

FIG. 20 is a flow diagram that illustrates the processing of the “applymedian filter” component of the imaging and detection system in someembodiments. In block 2001, the component calculates the median for eachpixel of the first filtered image frame for a certain number ofdown-track locations before and after the current down-track location.In block 2002, the component calculates the difference between eachpixel of the first filtered image frame and the median. In block 2003,the component identifies a neighborhood for each pixel of the firstfiltered image frame. In block 2004, the component identifies theminimum difference within the neighborhood of each pixel of the firstfiltered image frame. In block 2005, the component sets each pixel ofthe second filtered image frame to the value of the corresponding pixelof the first filtered image frame when the minimum difference is lessthan zero and to the median otherwise. The component then returns.

FIG. 21 is a flow diagram that illustrates the processing of the“restore spots” component of the imaging and detection system in someembodiments. The component inputs the second filtered image frame andoutputs the post-processed image frame. In block 2101, the componentcalculates the percentile value of the spot based on the pixels of thecorresponding clipped image frame. In block 2102, the componentinitializes the pixels of a temporary image frame to thesubtract-pixel-mean image frame. In block 2103, the component sets thepixels of the temporary image frame that are within the spot to thepercentile value plus one. In block 2104, the component selects a seedpixel within the spot and identifies a restored spot that includes thepixels of the original spot. In block 2105, the component sets eachpixel in the grown spot to its value in the subtract-pixel-mean imageframe; otherwise, the pixel is set to zero. The component then returns.

FIG. 22 is a flow diagram that illustrates the processing of the “detectsubsurface object” component of the imaging and detection system in someembodiments. In block 2201, the component invokes the “generatedetection statistics” component to generate the detection statistics forthe post-processed image frame. In block 2202, the component identifieswhether the detection statistics over a sequence of previous down-tracklocations include a peak indicating the presence of a subsurface objectat that down-track location.

FIG. 23 is a flow diagram that illustrates the processing of the“generate detection statistics” component of the imaging and detectionsystem in some embodiments. The component inputs the post-processedimage frame and outputs the detection statistic for a down-tracklocation. In block 2301, the component calculates the median of thereconstructed image frame. In block 2302, the component calculates thetotal energy of the pixels within the spot of the post-processed imageframe. In block 2303, the component calculates the detection statistic.In block 2304, the component adjusts the detection statistic based onthe area of the spot and then returns.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustrationbut that various modifications may be made without deviating from thescope of the invention. Accordingly, the invention is not limited exceptas by the appended claims.

I/we claim:
 1. A method performed by a computing device for suppressingeffects of extraneous signals in a current return signal acquired by areceiver from a signal emitted by a transmitter, the current returnsignal collected at a current cross-track location of a currentdown-track location, the method comprising: calculating an estimatedbackground signal for the current return signal based on a combinationof an estimated background signal for a prior return signal for a priordown-track location that has a similar air gap to the current returnsignal and an estimated gain and bias adjusted return signal for theprior return signal; calculating a gain and bias adjusted return signalfor the current return signal; and calculating an estimated foregroundsignal for the current return signal based on a difference between thegain and bias adjusted return signal for the current return signal andthe estimated background signal for the current return signal.
 2. Themethod of claim 1 including calculating an adaptation parameter based ondisparity between the gain and bias adjusted return signal and theestimated background signal for the prior return signal and wherein thecalculating of the estimated background signal for the current returnsignal weights the estimated background signal for a prior return signaland the estimated gain and bias adjusted return signal for the priorreturn signal based on the adaptation parameter.
 3. The method of claim2 wherein the adaptation parameter decreases with increasing disparitybetween the estimated background signal for the prior return signal andthe estimated gain and bias adjusted return signal for the prior returnsignal.
 4. The method of claim 2 wherein the disparity is a sum ofdifferences between the estimated background signal for the prior returnsignal and the estimated gain and bias adjusted return signal for theprior return signal.
 5. The method of claim 2 wherein the adaptationparameter is based on a mean of disparities of prior down-tracklocations and a standard deviation of disparities of prior down-tracklocations.
 6. The method of claim 1 wherein the air gap is a time delaybetween when the transmitter emits the signal and a return signal thatbounces off a surface is acquired by the receiver.
 7. The method ofclaim 1 wherein the transmitter and receiver are within a linear arrayof transceivers that operates in a multistatic mode.
 8. The method ofclaim 6 wherein emitted signals are emitted into a medium having asurface and including: for each of a plurality of down-track locations,for each of a plurality of cross-track locations for each down-tracklocation, calculating an estimated foreground signal for the returnsignal for that cross-track location; and detecting an object below thesurface of medium from the estimated foreground signals for the returnsignals.
 9. The method of claim 1 wherein the prior return signal wasacquired by the same receiver from a signal emitted by the sametransmitter.
 10. A computer-readable storage device storingcomputer-executable instructions for controlling a computing device tosuppress effects of extraneous signals in return signals acquired byreceivers from ground-penetrating radar signals emitted by transmittersoperating in a multistatic mode, by a method, the method comprising:receiving return signals for each transmitter and receiver pair at eachof a plurality of down-track locations; and for each current returnsignal, calculating an estimated background signal for the currentreturn signal based on an estimated background signal for a prior returnsignal for a prior down-track location that has a similar air gap to thecurrent return signal and an estimated gain and bias adjusted returnsignal for the prior return signal; calculating a gain and bias adjustedreturn signal for the current return signal; and calculating anestimated foreground signal for the current return signal based on adifference between the gain and bias adjusted return signal for thecurrent return signal and the estimated background signal for thecurrent return signal.
 11. The computer-readable storage device of claim10 including calculating an adaptation parameter based on disparitybetween the gain and bias adjusted return signal and the estimatedbackground signal for the prior return signal and wherein thecalculating of the estimated background signal for the current returnsignal weights the estimated background signal for a prior return signaland the estimated gain and bias adjusted return signal for the priorreturn signal based on the adaptation parameter.
 12. Thecomputer-readable storage device of claim 11 wherein the adaptationparameter decreases with increasing disparity between the estimatedbackground signal for the prior return signal and the estimated gain andbias adjusted return signal for the prior return signal.
 13. Thecomputer-readable storage device of claim 11 wherein the disparity is asum of differences between the estimated background signal for the priorreturn signal and the estimated gain and bias adjusted return signal forthe prior return signal.
 14. The computer-readable storage device ofclaim 11 wherein the adaptation parameter is based on a mean ofdisparities of prior down-track locations and a standard deviation ofdisparities of prior down-track locations.
 15. The computer-readablestorage device of claim 10 wherein emitted signals are emitted into amedium having a surface and including detecting an object below thesurface of the medium from the estimated foreground signals for thereturn signals.
 16. A computing device for suppressing effects ofextraneous signals in a current return signal acquired by a receiverfrom a signal emitted by a transmitter, the signal being aground-penetrating radar signal emitting towards the ground, the currentreturn signal having a plurality of samples corresponding to times atwhich the return signal was sampled by the receiver, the current returnsignal collected at current down-track location, comprising: a componentthat generates an adaptation parameter based on disparity between anestimated background signal for a prior return signal for a priordown-track location that has a similar time delay between when thetransmitter emits the signal and a return signal that bounces off asurface is acquired by the receiver as the current return signal and anestimated gain and bias adjusted return signal for the prior returnsignal; a component that generates an estimated background signal forthe current return signal as a combination of the estimated backgroundsignal for the prior return signal and the estimated gain and biasadjusted return signal for the prior return signal, the combinationbeing weighted by the adaptation parameter; a component that generates again and bias adjusted return signal for the current return signal; anda component that generates an estimated foreground signal for thecurrent return signal based on a difference between the gain and biasadjusted return signal for the current return signal and the estimatedbackground signal for the current return signal.
 17. The computingdevice of claim 16 wherein the adaptation parameter is based on a meanof disparities of prior down-track locations and a standard deviation ofdisparities of prior down-track locations.
 18. The computing device ofclaim 16 wherein the transmitter and receiver are within a linear arrayof transceivers that operates in a multistatic mode.
 19. The computingdevice of claim 18 including: a component that for each of a pluralityof down-track locations and for each of a plurality of cross-tracklocations for each down-track location, generates an estimatedforeground signal for the return signal for that cross-track location;and a component that detects a buried object from the estimatedforeground signals for the return signals.
 20. The computing device ofclaim 19 wherein the buried object is an explosive device.